Fix PHP 8.5 null array key creating a Dashboard Panel without layoutMode set
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/dashboard/layoutconfig/PhabricatorDashboardPanelRefList.php:16]
```
Closes T16431
Test Plan:
* PHP 8.5
* Go to http://phorge.localhost/dashboard/edit/form/default/
* Create a dashboard with "Layout" set to "Two Columns: 33%/66%"
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16431
Differential Revision: https://we.phorge.it/D26651