[wayland] avoid potential crash when checking for window inhibitions on desktop change

Summary:
Xwayland clients are sometimes offset from being visible to having a
surface applied.

We might also have internal windows which will be AbstractClients
without a surface.

No idle interface will be set up for non wayland clients, but on a
desktop change we itterate through all AbstractClients and need to guard
somewhere.

BUG: 420039

Test Plan: None

Reviewers: KWin, apol

Reviewed By: apol

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28858