don't pass User object when uid string is expected

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

lib/private/legacy/helper.php

@@ -558,7 +558,8 @@ class OC_Helper {
 				|| $storage->instanceOfStorage('\OC\Files\ObjectStore\HomeObjectStoreStorage')
 			) {
 				/** @var \OC\Files\Storage\Home $storage */
-				$user = $storage->getUser();
+				$userInstance = $storage->getUser();
+				$user = ($userInstance === null) ? null : $userInstance->getUID();
 			} else {
 				$user = \OC::$server->getUserSession()->getUser()->getUID();
 			}