choicesource doesn't throw error when no values is selected if widget is required

As in the case of bug #1062207, when a choice edit widget is a required field it should require some default value to render. The UI of the choice widget doesn't allow for a 'not selected' representation and so users think the first in the list if selected when in fact no value is set on the form.

The JS should either throw an error that there's no current value set or perhaps add in the "-None Selected-" type value for things that can be None/Null.