PHP 8.5: Fix null array key creating a milestone in PhabricatorProjectEditEngine
Summary:
Setting null as an array key is deprecated since PHP 8.5 per https://www.php.net/releases/8.5/en.php: "Using null as an array offset or when calling array_key_exists() is now deprecated. Use an empty string instead."
```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/project/engine/PhabricatorProjectEditEngine.php:128]
```
Closes T16504
Test Plan:
* PHP 8.5
* Go to an existing parent project
* Select "Subprojects" in the sidebar
* Click "Create Milestone" on the right
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16504
Differential Revision: https://we.phorge.it/D26765