Fix PHP 8.5 null array key creating a Dashboard Panel without layoutMode set master
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