Fix PHP 8.1 "strlen(null)" exception checking for open setup issues in PhabricatorSetupCheck

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/config/check/PhabricatorSetupCheck.php:125]
```

Closes T15823

Test Plan: Run `./bin/cache purge --all`, check any Phorge page in browser before and after applying this patch.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15823

Differential Revision: https://we.phorge.it/D25633

src/applications/config/check/PhabricatorSetupCheck.php

···
     $db_cache = new PhabricatorKeyValueDatabaseCache();
     try {
       $value = $db_cache->getKey('phabricator.setup.issue-keys');
-      if (!strlen($value)) {
+      if (!phutil_nonempty_string($value)) {
         return null;
       }
       return phutil_json_decode($value);