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

Author: vulcainman

src/applications/notification/client/PhabricatorNotificationServerRef.php

@@ -161,9 +161,9 @@ public function getURI($to_path = '') {
     return $uri;
   }
 
-  public function getWebsocketURI($to_path = null) {
+  public function getWebsocketURI($to_path = '') {
     $instance = PhabricatorEnv::getEnvConfig('cluster.instance');
-    if (strlen($instance)) {
+    if (phutil_nonempty_string($instance)) {
       $to_path = $to_path.'~'.$instance.'/';
     }