PHP 8.5: Avoid null array key in Conduit's diffusion.commit.edit
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/diffusion/xaction/DiffusionCommitAuditTransaction.php:35]
```
Closes T16437
Test Plan: On PHP 8.5, go to http://phorge.localhost/conduit/method/diffusion.commit.edit/
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16437
Differential Revision: https://we.phorge.it/D26648