popupMenu: Don't count the separator as an item when returning isEmpty

If we have a separator don't use it's possibly-unsynced visibility to
determine if the menu is empty or not.

https://bugzilla.gnome.org/show_bug.cgi?id=70386

js/ui/popupMenu.js

@@ -722,6 +722,8 @@ const PopupMenuBase = new Lang.Class({
 
     isEmpty: function() {
         let hasVisibleChildren = this.box.get_children().some(function(child) {
+            if (child._delegate instanceof PopupSeparatorMenuItem)
+                return false;
             return child.visible;
         });