notificationDamon: Always send the activation token

notificationDamon: Always send the activation token

In case of FDO notifications, we may receive a notification coming from
non-desktop-applications (e.g. gsd does that), but in any case when
activating an action we should always provide back the activation-token.

So send the startup notification ID in any case, even if the appId is
unset (in fact get_startup_notify_id() uses a nullable parameter for a
reason since GNOME/glib@6210708f).

In wayland this more than enough, while in X11 we could have used the
gnome-shell desktop file as fallback, but that's gone for good now!

Fixes: bb73bf5
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3891>