Fix PHP 8.1 "strlen(null)" exception which blocks rendering creation form pages for Legalpad, Macro, Settings

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 T15309

Test Plan:
Applied this change (on top of `D25144`, `D25145`, `D25146`, `D25147`, `D25150`,
`D25151`, `D25152`, `D25153`, `D25154`, `D25155`, `D25156`, `D25158` locally applied)
and the pages `/legalpad/edit/`, `/macro/create/`, `/settings/builtin/global/`
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: T15309

Differential Revision: https://we.phorge.it/D25159

src/applications/transactions/editengine/PhabricatorEditEngine.php

@@ -941,7 +941,7 @@ abstract class PhabricatorEditEngine
       }
     } else {
       $form_key = $request->getURIData('formKey');
-      if (strlen($form_key)) {
+      if (phutil_nonempty_string($form_key)) {
         $config = $this->loadEditEngineConfigurationWithIdentifier($form_key);
 
         if (!$config) {