Avoid crash without XWayland

Summary: readX11ShadowProperty fails without an X connection

Test Plan: Tested by Kai who didn't have xwayland

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21965