GraphQL: fix a bug when the class name isn't provided

Author: dunglas

src/GraphQl/Type/SchemaBuilder.php

@@ -366,6 +366,10 @@ private function convertType(Type $type, bool $input = false, string $mutationNa
                 }
 
                 $resourceClass = $this->isCollection($type) ? $type->getCollectionValueType()->getClassName() : $type->getClassName();
+                if (null === $resourceClass) {
+                    return null;
+                }
+
                 try {
                     $resourceMetadata = $this->resourceMetadataFactory->create($resourceClass);
                     if ([] === $resourceMetadata->getGraphql() ?? []) {