fix(approval): posts approved for deleted users error

In certain edge cases posts get approved through moderative action and
throws an error when the user has already been deleted.

Ref: DISCUSS-11K on sentry

extensions/approval/src/Listener/ApproveContent.php

@@ -69,7 +69,9 @@ class ApproveContent
 
         $discussion->save();
 
+        if ($discussion->user) {
             $user->refreshCommentCount();
             $user->save();
         }
+    }
 }