If the initiator no longer exists return UID

It can happen that the share initiator is deleted. Because of the new
resharing behaviour this share then still exists. We just can fetch the
displayname properly.

apps/files_sharing/api/share20ocs.php

@@ -91,7 +91,7 @@ class Share20OCS {
 			'id' => $share->getId(),
 			'share_type' => $share->getShareType(),
 			'uid_owner' => $share->getSharedBy(),
-			'displayname_owner' => $sharedBy->getDisplayName(),
+			'displayname_owner' => $sharedBy !== null ? $sharedBy->getDisplayName() : $share->getSharedBy(),
 			'permissions' => $share->getPermissions(),
 			'stime' => $share->getShareTime()->getTimestamp(),
 			'parent' => null,