notification: Use fdo notifictions for applications without IDs.

The GTK notifcation system (i.e. GNOME Shell) doesn't support notifcations without an ID and silently ignores them.
In this case, use the fdo notification system so these show.

src/notification.c

@@ -228,6 +228,7 @@ handle_add_notification (XdpImplNotification *object,
                          GVariant *arg_notification)
 {
   if (gtk_notifications == NULL ||
+      g_str_equal (arg_app_id, "") ||
       g_dbus_proxy_get_name_owner (G_DBUS_PROXY (gtk_notifications)) == NULL ||
       has_unprefixed_action (arg_notification))
     handle_add_notification_fdo (object, invocation, arg_app_id, arg_id, arg_notification);