Trying to get the xwindow of a wayland only window would fail when
casting to a x11 window. Which happens as
meta_x11_display_set_input_focus is called whenever the focused
window changes, whether it is a wayland or x11 one
Fixes: bc9cd123e ("window: Move xwindow to WindowX11")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3506>