fix standards

Author: denissonleal

src/Jenssegers/Mongodb/Queue/Failed/MongoFailedJobProvider.php

@@ -47,7 +47,7 @@ public function find($id)
         $job = $this->getTable()->find($id);
 
         if (!$job) {
-            return null;
+            return;
         }
 
         $job['id'] = (string) $job['_id'];