wayland: xdg-activation: Don't assume there's a focus surface

wayland: xdg-activation: Don't assume there's a focus surface

Tools like gtk4-launch can't set surface on the activation token so
don't require it. If the compositor requires it we can't do anything
about it anyway. This avoids a critical:

   (gtk4-launch:23497): Gdk-CRITICAL **: 17:07:24.704: gdk_wayland_surface_get_wl_surface: assertion 'GDK_IS_WAYLAND_SURFACE (surface)' failed

Fixes: be4216e051 ("gdk/wayland: Support the xdg-activation wayland protocol")
Signed-off-by: Guido Günther <agx@sigxcpu.org>