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