ensure correct return type, fixes #2765

Closes #2765

Merge request studip/studip!1881

lib/classes/Request.class.php

@@ -170,7 +170,7 @@ class Request implements ArrayAccess, IteratorAggregate
             '',
             urldecode(self::path())
         );
-        return parse_url($path_info, PHP_URL_PATH);
+        return parse_url($path_info, PHP_URL_PATH) ?? '';
     }
 
     /**