Formulator: Do not add empty item if item_list is empty for SingleItemsWidget and MultiItemsWidget

we used to add this value because select without option (that happens in MultiListField only) is invalid on XHTML.

But it was not at all required for other cases like MultiCheckboxField or RadioField.

Also, select without option is no longer invalid on HTML5.

Considering that showing dummy unselectable is anyway meaningless for users, we drop it for all cases.