[wayland] Squash reconfigure methods

Summary:
We can handle the case of a null size inside
ShellClient::requestGeometry and that makes that method more robust.

With that we don't need to handle separate code paths in
the handler of RequestGeometryBlocker.

It's not an exact identical code path, but everything still works out.

If the geometry is unchanged, we'll save the positionAfterResize but
when we apply it doSetGeometry will no-op.

There's also an assumption that toggling maximised will always send a
valid size, but that's true for the current state.

Test Plan: Ran unit tests

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D19331