fixing issue when there is no ugroups
Merge commit 'refs/changes/45/345/1' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  fixing issue when there is no ugroups

Conflicts:
	plugins/tracker/include/Tracker/FormElement/Tracker_FormElement_Field_List_Bind_Users.class.php