Fix PHP 8.1 "strlen(null)" exception for empty default branch in PhabricatorRepository

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/repository/storage/PhabricatorRepository.php:2785]
```

Closes T16524

Test Plan:
1. PHP 8.1 or higher
2. Go to http://phorge.localhost/conduit/method/diffusion.repository.search/
3. In the "Attachments" field, enter `{"metrics": true}`
4. Click "Call Method"

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16524

Differential Revision: https://we.phorge.it/D26795

src/applications/repository/storage/PhabricatorRepository.php

···
     $permanent_rules = $this->getStringListForConduit($permanent_rules);
 
     $default_branch = $this->getDefaultBranch();
-    if (!strlen($default_branch)) {
+    if (!phutil_nonempty_string($default_branch)) {
       $default_branch = null;
     }