According to the HTML spec per the content model for the option element, neither <option></option> nor <option label=""></option> are valid.

I believe they should be valid, because:

1) there is widespread use of <option></option> on the internet, and

2) A (granted, non-normative) section of the select element describes setting the length attribute of a select element to add "new blank option elements to the select".  There is no other definition of "blank option" and all current web browsers will generate "<option></option>" in response to increasing a select's length.