Fix for issue #21510

Author: ccasciotti

app/code/Magento/Indexer/Model/Indexer.php

@@ -361,7 +361,7 @@ public function getLatestUpdated()
                 return $this->getView()->getUpdated();
             }
         }
-        return $this->getState()->getUpdated();
+        return $this->getState()->getUpdated() ?: '';
     }
 
     /**

app/code/Magento/Indexer/Test/Unit/Model/IndexerTest.php

@@ -164,7 +164,12 @@ public function testGetLatestUpdated($getViewIsEnabled, $getViewGetUpdated, $get
                 }
             }
         } else {
-            $this->assertEquals($getStateGetUpdated, $this->model->getLatestUpdated());
+            $actualGetLatestUpdated = $this->model->getLatestUpdated();
+            $this->assertEquals($getStateGetUpdated, $actualGetLatestUpdated);
+
+            if ($getStateGetUpdated === null) {
+                $this->assertNotNull($actualGetLatestUpdated);
+            }
         }
     }
 
@@ -182,7 +187,8 @@ public function getLatestUpdatedDataProvider()
             [true, '', '06-Jan-1944'],
             [true, '06-Jan-1944', ''],
             [true, '', ''],
-            [true, '06-Jan-1944', '05-Jan-1944']
+            [true, '06-Jan-1944', '05-Jan-1944'],
+            [false, null, null],
         ];
     }