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 nexedi/erp5!777