Nullptr check for rootInfo when setting active client

Summary:
Preparation for X free KWin. Code is called for both X11 and Wayland
windows. So make it not crash if we would not have an X server.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D6908