Don't crash if a decoration doesn't get created

Summary:
With the NoBorder option set the DecorationBridge won't create
decorations. Thus we get a nullptr and obviously should not call
into it. There was already a check for whether decoration is null,
so that is a rather embarrassing bug.

Test Plan: Test case added which exposes the crash

Reviewers: Plasma on Wayland, KWin

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D1980