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

js/ui/messageTray.js

@@ -438,6 +438,11 @@ const Notification = new Lang.Class({
         this._bannerLabel = this._bannerUrlHighlighter.actor;
         this._bannerBox.add_actor(this._bannerLabel);
 
+        // If called with only one argument we assume the caller
+        // will call .update() later on. This is the case of
+        // NotificationDaemon, which wants to use the same code
+        // for new and updated notifications
+        if (arguments.length != 1)
             this.update(title, banner, params);
     },