(legacy) fix key check

(cherry picked from commit debe657e)

src/etc/rc.filter_synchronize

@@ -200,7 +200,7 @@ function carp_sync_xml($url, $username, $password, $sections, $method = 'opnsens
 
     // when syncing users, send last used uid/gid over
     if (in_array('system.user', $sections)) {
-        if (!in_array('system', $transport_data)) {
+        if (!isset($transport_data['system'])) {
             $transport_data['system'] = array();
         }
         $transport_data['system']['nextuid'] = $config['system']['nextuid'];