[qpa] Always keep a at least one screen

Summary:
Qt does not like having no screens. Both Qt XCB and Wayland QPAs have
systems pretend there's always at least 1 screen
present.

Kwin already uses a dummy screen on startup, this patch reinserts the
dummy screen if the platform states that all real screens are removed.

BUG: 399564

Test Plan:
Logged in, everything still worked
Did not reproduce the original bug

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16096