Don't try to filter null key combinations

Summary:
When using composite key combinations, kwin would do random weird
actions when the first key was pressed (e.g. ` key). This makes sure we
are not trying to match.

BUG: 390110

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: graesslin, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12416