NotificationDaemon: fix icons for notifications without an app

NotificationDaemon doesn't pass a gicon to the Notification constructor,
because it calls .update() immediately after, so messageTray.js
calls into Source.createIcon(), which returns null and crashes.
Instead, shortcut the Notification constructor by skipping
.update() completely.

https://bugzilla.gnome.org/show_bug.cgi?id=709998