Fix PHP 8.1 "strlen(null)" exception which blocks rendering Notifications' Advanced Search page

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Closes T15320

Test Plan: Applied this change and `/notification/query/advanced/` rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15320

Differential Revision: https://we.phorge.it/D25169

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

@@ -872,7 +872,7 @@ abstract class PhabricatorApplicationSearchEngine extends Phobject {
   protected function readBoolFromRequest(
     AphrontRequest $request,
     $key) {
-    if (!strlen($request->getStr($key))) {
+    if (!phutil_nonempty_string($request->getStr($key))) {
       return null;
     }
     return $request->getBool($key);