Fix PHP 8.1 "preg_replace(null)" exception in PhabricatorEditorURIEngine.php
Summary:
Passing `null` to `preg_replace()` is deprecated behavior since PHP 8.1.
Thus check if `$repository->getRepositorySlug()` is set before passing that to `escapeToken()`.
```
ERROR 8192: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated at [<phorge>/src/infrastructure/editor/PhabricatorEditorURIEngine.php:283]
```
Closes T15824
Test Plan: Unknown.
Reviewers: O1 Blessed Committers, chris
Reviewed By: O1 Blessed Committers, chris
Subscribers: chris, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15824
Differential Revision: https://we.phorge.it/D26036