Redirect from component override broken when adding via theme settings form

Problem/Motivation

When adding an override via the theme settings form, the redirect leads to a blank page instead of getting redirected back to the theme settings form. This happens because the current code in ComponentOverrideForm::ajaxSubmit() checks whether there is a redirect destination, which is set, but is not correct when submitting the form coming from the theme settings.

Steps to reproduce

Go to the theme settings form and add an override via the local action link.

Proposed resolution

Check whether the destination query parameter is actually set or not and redirect back to the theme settings in case it is not.