[HttpFoundation] Fix TypeError on null $_SESSION in NativeSessionStorage::save()

Author: chalasr

src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php

@@ -256,7 +256,7 @@ public function save()
                 unset($_SESSION[$key]);
             }
         }
-        if ([$key = $this->metadataBag->getStorageKey()] === array_keys($_SESSION)) {
+        if ($_SESSION && [$key = $this->metadataBag->getStorageKey()] === array_keys($_SESSION)) {
             unset($_SESSION[$key]);
         }
 

src/Symfony/Component/HttpFoundation/Tests/Session/Storage/NativeSessionStorageTest.php

@@ -321,4 +321,13 @@ public function testRegenerateInvalidSessionIdForNativeFileSessionHandler()
         $this->assertTrue($started);
         $this->assertSame('&~[', session_id());
     }
+
+    public function testSaveHandlesNullSessionGracefully()
+    {
+        $storage = $this->getStorage();
+        $_SESSION = null;
+        $storage->save();
+
+        $this->addToAssertionCount(1);
+    }
 }