shell: Handle wl_shell surfaces created by xwayland correctly

shell: Handle wl_shell surfaces created by xwayland correctly

When xwayland creates a shell surface we don't have a resource.  The
recently added shell_surface_is_wl_shell/xdg_surface() tests don't
handle that very well.

For now, we assume that a surface without a resource is created from
xwayland and is a wl_shell surface.  We'll want to modify that to be a
xdg surface eventually, but for now this stops weston from crashing.