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.