Fix PHP 8.1 "strlen(null)" exception which blocks rendering Config's Module 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 T15308

Test Plan:
Applied this change (on top of `D25144`, `D25145`, `D25146`, `D25147`, `D25150`,
`D25151`, `D25152`, `D25153`, `D25154`, `D25155`) and `/config/module/` rendered
in web browser.

T15308

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15308

Differential Revision: https://we.phorge.it/D25158

src/applications/config/controller/module/PhabricatorConfigModuleController.php

@@ -9,7 +9,7 @@ final class PhabricatorConfigModuleController
 
     $all_modules = PhabricatorConfigModule::getAllModules();
 
-    if (!strlen($key)) {
+    if (!phutil_nonempty_string($key)) {
       $key = head_key($all_modules);
     }