Ensure that url suffix resolvers return strings to match function declaration

app/code/Magento/CatalogUrlRewriteGraphQl/Model/Resolver/CategoryUrlSuffix.php

@@ -75,7 +75,7 @@ private function getCategoryUrlSuffix(int $storeId): string
                 self::$xml_path_category_url_suffix,
                 ScopeInterface::SCOPE_STORE,
                 $storeId
-            );
+            ) ?? '';
         }
         return $this->categoryUrlSuffix[$storeId];
     }

app/code/Magento/CatalogUrlRewriteGraphQl/Model/Resolver/ProductUrlSuffix.php

@@ -75,7 +75,7 @@ private function getProductUrlSuffix(int $storeId): string
                 self::$xml_path_product_url_suffix,
                 ScopeInterface::SCOPE_STORE,
                 $storeId
-            );
+            ) ?? '';
         }
         return $this->productUrlSuffix[$storeId];
     }

app/code/Magento/CatalogUrlRewriteGraphQl/Test/Unit/Model/Resolver/CategoryUrlSuffixTest.php

@@ -0,0 +1,173 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\CatalogUrlRewriteGraphQl\Test\Unit\Model\Resolver;
+
+use Magento\CatalogUrlRewriteGraphQl\Model\Resolver\CategoryUrlSuffix;
+use Magento\Framework\GraphQl\Config\Element\Field;
+use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
+use Magento\GraphQl\Model\Query\ContextExtensionInterface;
+use Magento\GraphQl\Model\Query\ContextInterface;
+use Magento\Store\Api\Data\StoreInterface;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use Magento\Framework\App\Config\ScopeConfigInterface;
+
+/**
+ * Test for \Magento\CatalogUrlRewriteGraphQl\Model\Resolver\CategoryUrlSuffix.
+ */
+class CategoryUrlSuffixTest extends TestCase
+{
+    /**
+     * @var ScopeConfigInterface|MockObject
+     */
+    private $scopeConfigMock;
+
+    /**
+     * @var ContextInterface|MockObject
+     */
+    private $contextMock;
+
+    /**
+     * @var ContextExtensionInterface|MockObject
+     */
+    private $contextExtensionMock;
+
+    /**
+     * @var StoreInterface|MockObject
+     */
+    private $storeMock;
+
+    /**
+     * @var Field|MockObject
+     */
+    private $fieldMock;
+
+    /**
+     * @var ResolveInfo|MockObject
+     */
+    private $resolveInfoMock;
+
+    /**
+     * @var CategoryUrlSuffix
+     */
+    private $resolver;
+
+    /**
+     * @inheritDoc
+     */
+    protected function setUp(): void
+    {
+        $this->contextMock = $this->getMockBuilder(ContextInterface::class)
+            ->disableOriginalConstructor()
+            ->setMethods(
+                [
+                    'getExtensionAttributes'
+                ]
+            )
+            ->getMockForAbstractClass();
+
+        $this->contextExtensionMock = $this->getMockBuilder(ContextExtensionInterface::class)
+            ->setMethods(
+                [
+                    'getStore'
+                ]
+            )
+            ->getMockForAbstractClass();
+
+        $this->storeMock = $this->getMockBuilder(StoreInterface::class)
+            ->setMethods(
+                [
+                    'getId'
+                ]
+            )
+            ->getMockForAbstractClass();
+
+        $this->fieldMock = $this->getMockBuilder(Field::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $this->resolveInfoMock = $this->getMockBuilder(ResolveInfo::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $this->scopeConfigMock = $this->getMockBuilder(ScopeConfigInterface::class)
+            ->getMockForAbstractClass();
+
+        $this->resolver = new CategoryUrlSuffix(
+            $this->scopeConfigMock
+        );
+    }
+
+    /**
+     * Verify that empty string is returned when config value is null
+     */
+    public function testNullValue()
+    {
+        $this->scopeConfigMock->expects($this->once())
+            ->method('getValue')
+            ->willReturn(null);
+
+        $this->contextMock
+            ->expects($this->once())
+            ->method('getExtensionAttributes')
+            ->willReturn($this->contextExtensionMock);
+
+        $this->contextExtensionMock
+            ->expects($this->once())
+            ->method('getStore')
+            ->willReturn($this->storeMock);
+
+        $this->storeMock
+            ->expects($this->once())
+            ->method('getId')
+            ->willReturn(1);
+
+        $this->assertEquals(
+            '',
+            $this->resolver->resolve(
+                $this->fieldMock,
+                $this->contextMock,
+                $this->resolveInfoMock
+            )
+        );
+    }
+
+    /**
+     * Verify that the configured value is returned
+     */
+    public function testNonNullValue()
+    {
+        $value = 'html';
+        $this->scopeConfigMock->expects($this->once())
+            ->method('getValue')
+            ->willReturn($value);
+
+        $this->contextMock
+            ->expects($this->once())
+            ->method('getExtensionAttributes')
+            ->willReturn($this->contextExtensionMock);
+
+        $this->contextExtensionMock
+            ->expects($this->once())
+            ->method('getStore')
+            ->willReturn($this->storeMock);
+
+        $this->storeMock
+            ->expects($this->once())
+            ->method('getId')
+            ->willReturn(1);
+
+        $this->assertEquals(
+            $value,
+            $this->resolver->resolve(
+                $this->fieldMock,
+                $this->contextMock,
+                $this->resolveInfoMock
+            )
+        );
+    }
+}

app/code/Magento/CatalogUrlRewriteGraphQl/Test/Unit/Model/Resolver/ProductUrlSuffixTest.php

@@ -0,0 +1,173 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\CatalogUrlRewriteGraphQl\Test\Unit\Model\Resolver;
+
+use Magento\CatalogUrlRewriteGraphQl\Model\Resolver\ProductUrlSuffix;
+use Magento\Framework\GraphQl\Config\Element\Field;
+use Magento\Framework\GraphQl\Schema\Type\ResolveInfo;
+use Magento\GraphQl\Model\Query\ContextExtensionInterface;
+use Magento\GraphQl\Model\Query\ContextInterface;
+use Magento\Store\Api\Data\StoreInterface;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use Magento\Framework\App\Config\ScopeConfigInterface;
+
+/**
+ * Test for \Magento\CatalogUrlRewriteGraphQl\Model\Resolver\ProductUrlSuffix.
+ */
+class ProductUrlSuffixTest extends TestCase
+{
+    /**
+     * @var ScopeConfigInterface|MockObject
+     */
+    private $scopeConfigMock;
+
+    /**
+     * @var ContextInterface|MockObject
+     */
+    private $contextMock;
+
+    /**
+     * @var ContextExtensionInterface|MockObject
+     */
+    private $contextExtensionMock;
+
+    /**
+     * @var StoreInterface|MockObject
+     */
+    private $storeMock;
+
+    /**
+     * @var Field|MockObject
+     */
+    private $fieldMock;
+
+    /**
+     * @var ResolveInfo|MockObject
+     */
+    private $resolveInfoMock;
+
+    /**
+     * @var ProductUrlSuffix
+     */
+    private $resolver;
+
+    /**
+     * @inheritDoc
+     */
+    protected function setUp(): void
+    {
+        $this->contextMock = $this->getMockBuilder(ContextInterface::class)
+            ->disableOriginalConstructor()
+            ->setMethods(
+                [
+                    'getExtensionAttributes'
+                ]
+            )
+            ->getMockForAbstractClass();
+
+        $this->contextExtensionMock = $this->getMockBuilder(ContextExtensionInterface::class)
+            ->setMethods(
+                [
+                    'getStore'
+                ]
+            )
+            ->getMockForAbstractClass();
+
+        $this->storeMock = $this->getMockBuilder(StoreInterface::class)
+            ->setMethods(
+                [
+                    'getId'
+                ]
+            )
+            ->getMockForAbstractClass();
+
+        $this->fieldMock = $this->getMockBuilder(Field::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $this->resolveInfoMock = $this->getMockBuilder(ResolveInfo::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $this->scopeConfigMock = $this->getMockBuilder(ScopeConfigInterface::class)
+            ->getMockForAbstractClass();
+
+        $this->resolver = new ProductUrlSuffix(
+            $this->scopeConfigMock
+        );
+    }
+
+    /**
+     * Verify that empty string is returned when config value is null
+     */
+    public function testNullValue()
+    {
+        $this->scopeConfigMock->expects($this->once())
+            ->method('getValue')
+            ->willReturn(null);
+
+        $this->contextMock
+            ->expects($this->once())
+            ->method('getExtensionAttributes')
+            ->willReturn($this->contextExtensionMock);
+
+        $this->contextExtensionMock
+            ->expects($this->once())
+            ->method('getStore')
+            ->willReturn($this->storeMock);
+
+        $this->storeMock
+            ->expects($this->once())
+            ->method('getId')
+            ->willReturn(1);
+
+        $this->assertEquals(
+            '',
+            $this->resolver->resolve(
+                $this->fieldMock,
+                $this->contextMock,
+                $this->resolveInfoMock
+            )
+        );
+    }
+
+    /**
+     * Verify that the configured value is returned
+     */
+    public function testNonNullValue()
+    {
+        $value = 'html';
+        $this->scopeConfigMock->expects($this->once())
+            ->method('getValue')
+            ->willReturn($value);
+
+        $this->contextMock
+            ->expects($this->once())
+            ->method('getExtensionAttributes')
+            ->willReturn($this->contextExtensionMock);
+
+        $this->contextExtensionMock
+            ->expects($this->once())
+            ->method('getStore')
+            ->willReturn($this->storeMock);
+
+        $this->storeMock
+            ->expects($this->once())
+            ->method('getId')
+            ->willReturn(1);
+
+        $this->assertEquals(
+            $value,
+            $this->resolver->resolve(
+                $this->fieldMock,
+                $this->contextMock,
+                $this->resolveInfoMock
+            )
+        );
+    }
+}