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 <option> is anyway meaningless for users,
we drop it for all cases.

/reviewed-on !777

product/Formulator/Widget.py

@@ -839,9 +839,7 @@ class SingleItemsWidget(ItemsWidget):
     cell = getattr(REQUEST, 'cell', None)
     items = field.get_value('items', REQUEST=REQUEST, cell=cell)
     if not items:
-      # single item widget should have at least one child in order to produce
-      # valid XHTML; keep it enabled otherwise field validator always fails
-      return [self.render_item('', '', '', '', '')]
+      return []
 
     # check if we want to select first item
     if not value and field.get_value('first_item', REQUEST=REQUEST,
@@ -981,12 +979,8 @@ class MultiItemsWidget(ItemsWidget):
 
     items = field.get_value('items', REQUEST=REQUEST, cell=getattr(REQUEST, 'cell', None)) # Added request
     from Products.ERP5Form.MultiLinkField import MultiLinkFieldWidget
-    if not items and not isinstance(self, MultiLinkFieldWidget):
-      # multi items widget should have at least one child in order to produce
-      # valid XHTML; disable it so user can not select it.
-      # This cannot be applied to MultiLinkFields, which are just some <a>
-      # links
-      return [self.render_item('', '', '', '', 'disabled="disabled"')]
+    if not items:
+      return []
 
     css_class = field.get_value('css_class')
     extra_item = field.get_value('extra_item')