Xkb events should be handled by clutter backend but they are not translated
into an actual clutter event. However we're now handling them and also trying
to push an empty event to clutter queue, causing a critical error.
So in such case, just handle the native event but don't push the non-populated
clutter-event to the queue.
Fixes https://gitlab.gnome.org/GNOME/mutter/issues/750https://gitlab.gnome.org/GNOME/mutter/merge_requests/764