Forward showing desktop only to rootInfo if it changed

Summary:
There was a property change for whenever a window gets activated although
that doesn't change the state.

In addition a nullptr check for rootInfo is added for the future no
XWayland case.

Test Plan:
run xev -root to verify the property does not get updated
needlessly

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7097