Add tests for building JSON Schema of non-resource class

Author: teohhanhui

SchemaFactory.php

@@ -238,7 +238,7 @@ private function getMetadata(string $className, string $type = Schema::TYPE_OUTP
         if (!$this->isResourceClass($className)) {
             return [
                 null,
-                $serializerContext,
+                $serializerContext ?? [],
                 $className,
             ];
         }