Fix PHP 8.1 "strlen(null)" exception which blocks rendering Configuration's Settings 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 T15324

Test Plan: Applied this change and `/config/settings/` 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: T15324

Differential Revision: https://we.phorge.it/D25171

src/applications/config/controller/settings/PhabricatorConfigSettingsListController.php

@@ -7,7 +7,7 @@ final class PhabricatorConfigSettingsListController
     $viewer = $request->getViewer();
 
     $filter = $request->getURIData('filter');
-    if (!strlen($filter)) {
+    if (!phutil_nonempty_string($filter)) {
       $filter = 'settings';
     }