PHP 8.5: Avoid null array key creating a Harbormaster Build Plan
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/harbormaster/plan/HarbormasterBuildPlanBehavior.php:130]
```
Closes T16430
Test Plan: On PHP 8.5, go to http://phorge.localhost/harbormaster/plan/edit/form/default/
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16430
Differential Revision: https://we.phorge.it/D26650