Fix /dashboard/adjust/remove/ PHP 8.1 strlen(null) errors

Summary:
/dashboard/adjust/remove/ POST requests fail under PHP 8.1 with strlen(null) errors.

This change fixes it.

Fixes T15549

Test Plan:
View a dashboard which has some panels. Eg https://my.phorge.site/dashboard/view/1/
Click on a 'Manage Panel' dropdown and click on 'Remove Panel'

Confirm the panel is removed and no strlen(null) errors are generated.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15549

Differential Revision: https://we.phorge.it/D25345