Bugfix: Avoid implosion of empty <option>s

When using select2 on

<select>
  <option value="empty"></option>
  <option>Val 1</option>
  <option>Val 2</option>
</select>

The top option will not have the correct height.
Merge pull request #1042 from JensRantil/fix-issue-1042

Bugfix: Avoid implosion of empty <option>s