If converting any select to a dropdown, for example

<select name="someselect"
    <option value="1" selected="selected">A</option>
    <option value="2">B</option>
    <option value="3">C</option>
</select>

The select is not required, so an empty selection is valid. We already have a value selected (given by something else). But it is not possible to select an empty value from the generated dropdown.