Fix PHP 8.1 "trim(null)" exception in project.create for missing name
Summary:
Passing `null` to `trim()` is deprecated since PHP 8.1.
Convert null to an empty string to avoid complaints about trim, strlen, and PhabricatorSlug::isValidProjectSlug() expecting strings.
```
ERROR 8192: trim(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/project/xaction/PhabricatorProjectNameTransaction.php:75]
```
Closes T16420
Test Plan:
* PHP 8.1+
* Go to Go to http://phorge.localhost/conduit/method/project.create/
* Press the "Call Method" button
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16420
Differential Revision: https://we.phorge.it/D26649