Conduit: Fix PHP 8.1 "preg_replace(null)" exceptions in phriction.{edit|info}
Summary:
Passing null to `preg_match()` is deprecated since PHP 8.1.
```
ERROR 8192: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/util/PhabricatorSlug.php:17]
```
Closes T16424
Test Plan:
* PHP 8.1+
* Go to http://phorge.localhost/conduit/method/phriction.edit/ and press the "Call Method" button
* Go to http://phorge.localhost/conduit/method/phriction.info/ and press the "Call Method" button
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16424
Differential Revision: https://we.phorge.it/D26635