Check the background is valid when choosing background type

Summary:
I rushed in an emergency fix for 5.9.1 that added support for setting
the background type, which was a new property someone added to the
breeze theme in 5.9.1.

My last patch doesn't support clearing the background properly.
Effectively the old code would always set the type to "image".

BUG: 376484

Test Plan:
Set background to image. Checked config
Cleared background. Checked config

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4611