revert part of a8c80c52, re studip/studip#4578

lib/classes/Visibility.php

@@ -500,8 +500,8 @@ class Visibility
      */
     private static function getUser(&$user)
     {
-        if (!$user && User::findCurrent()) {
-            $user = User::findCurrent()->id;
+        if ($user == null) {
+            $user = $GLOBALS['user']->user_id;
         }
     }