We were doing our initial "empty" commit on the subsurface instead of the toplevel surface. As an incidence, we should not have received a configure event ever, not just on mutter. This fixes the following warning when using mutter compositor (aka gnome-shell):
waylandsink wlwindow.c:304:gst_wl_window_new_toplevel: The compositor did not send configure event.
cc @gkiagia