Correctly check forcebackgroundcontrastrule

Summary:
WindowBackgroundContrastRole property is a bool or undefined.

QVariant::isValid() returns true for QVariant(false) which is not what
we want for this code.

toBool on an invalid QVariant will return false.

Test Plan:
NA. Was based on code review.
To get a bug you'd have to PresentWindows/Slide effect manipulate a sliding popup, which
AFAIK doesn't happen.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13478