panel: Relax check for existing signal handler

Object.prototype.hasOwnProperty() is more precise than checking for
falsiness, for instance the following is true:

  { foo: undefined }.hasOwnProperty('foo');

However when checking for a handler ID, a more relaxed check is more
appropriate, as particularly 0 is not a valid handler ID.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/626

js/ui/panel.js

@@ -1167,7 +1167,7 @@ class Panel extends St.Widget {
     }
 
     _onMenuSet(indicator) {
-        if (!indicator.menu || indicator.menu.hasOwnProperty('_openChangedId'))
+        if (!indicator.menu || indicator.menu._openChangedId)
             return;
 
         indicator.menu._openChangedId = indicator.menu.connect('open-state-changed',