system: Hide the AltSwitcher when we have nothing to show

https://bugzilla.gnome.org/show_bug.cgi?id=706852

js/ui/status/system.js

@@ -60,6 +60,8 @@ const AltSwitcher = new Lang.Class({
             // sync the pointer to make sure they update.
             global.sync_pointer();
         }
+
+        this.actor.visible = (childToShow != null);
     },
 
     _onDestroy: function() {