Fix post update

exam/classes/Models/Post/Post.php

@@ -141,7 +141,7 @@ class Post implements JsonSerializable
         } else $reason = ["{$e->getCode()}"];
       }
     }
-    if ($failed !== null) {
+    if (count($failed) > 0) {
       $db->rollBack();
 
       throw ApiError::failedUpdate($failed, $reason);