Fix a PHP 8.1/8.2 deprecated use of strlen a NULL argument

Summary: This commit harmonizes PhabricatorNotificationServerRef's getURI and getWebsocketURI methods as discussed in D25382.

Test Plan: This is hard to say since I didn't know how/when this function might be called.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25401