Stick to the actual user id when available

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/Controller/WopiController.php

@@ -185,7 +185,7 @@ public function checkFileInfo(string $fileId, string $access_token): JSONRespons
 				'BaseFileName' => $file->getName(),
 				'Size' => $file->getSize(),
 				'Version' => $version,
-				'UserId' => !$wopi->isGuest() ? $wopi->getEditorUid() : $guestUserId,
+				'UserId' => !empty($wopi->getEditorUid()) ? $wopi->getEditorUid() : $guestUserId,
 				'OwnerId' => $wopi->getOwnerUid(),
 				'UserFriendlyName' => $userDisplayName,
 				'UserExtraInfo' => [],