optgroup選択リストの最後に空を追加します。

 <select>
  <option selected="" disabled="">Select a value</option>
  <option>Grumpy wizards make toxic brew for the evil Queen and Jack</option>
  <option>Quirky spud boys can jam after zapping five worthy Polysixes</option>
  <option>The wizard quickly jinxed the gnomes before they vaporized</option>
  <option>All questions asked by five watched experts amaze the judge</option>
  <optgroup label=""></optgroup>
 </select>