panel: don't break when indicator has no menu

Status indicators don't have necessarily a menu. If they have not we don't
want to add the menu in the PopupMenuManager.

https://bugzilla.gnome.org/show_bug.cgi?id=678694

js/ui/panel.js

@@ -1126,6 +1126,7 @@ const Panel = new Lang.Class({
         if (!position)
             position = 0;
         this._insertStatusItem(indicator.actor, position);
+        if (indicator.menu)
             this._menus.addMenu(indicator.menu);
 
         this._statusArea[role] = indicator;