xdg-shell: Don't early out of role commit before calling parent impl

Make sure to always call the parent role commit vfunc, so that they can
handle updating their state properly.

This means other places need to handle the situation where
surface->window is NULL on commit. This may for example happen when the
parent of a modal dialog is unmapped or NULL is attached to a
wl_shell_surface.

https://bugzilla.gnome.org/show_bug.cgi?id=769936