Relax assertion to include null as possible outcome (#9355)

Author: derrabus

lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php

@@ -95,7 +95,7 @@ protected function onNotFoundMetadata($className)
 
         $this->evm->dispatchEvent(Events::onClassMetadataNotFound, $eventArgs);
         $classMetadata = $eventArgs->getFoundMetadata();
-        assert($classMetadata instanceof ClassMetadata);
+        assert($classMetadata instanceof ClassMetadata || $classMetadata === null);
 
         return $classMetadata;
     }