Remove unnecessary return statement in slack.php

The return statement was redundant as the function naturally concludes without it. Removing it improves code clarity and eliminates dead code.

services/nginx/app/classes/slack.php

@@ -43,7 +43,6 @@ class slack implements notification_i
             if (!empty($webhook)) {
                 redis->cache_department_webhook($department_id, $webhook);
             }
-            return null;
         }
         return redis->get_department_webhook($department_id);
     }