[wayland] Check presence of the inhibitor object when a client is registered in IdleInhibition

Summary:
Some applications are not able to inhibit the idle behavior because
each of them creates an inhibitor object before the corresponding
ShellClient object becomes ready for painting.

BUG: 401499
FIXED-IN: 5.15.0

Test Plan: idle-inhibit client (from wlroots/examples) works.

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17227