Save dialog: Don't try to pass in undefined checkboxes fields to setupCheckboxes
checkboxFields in the setup data is optional, it's only actually set up by
DesktopArticleTarget's overridden openSaveDialog function, not mobile's
(or presumably other Target classes).
Bug: T109501
Change-Id: Ibc5f821be822fa3704bbfb55345687dd7795e203