Fix for select fields with 'multiple' option and no selected values

In
    
    <form>
      <select multiple="multiple" id="foobar" name="foobar">
        <option value="foo">Foo</option>
        <option value="bar">Bar</option>
      </select>
    </form>

Using

    $('form').areYouSure();

Leads to

    Uncaught TypeError: Cannot read property 'toString' of null jquery.are-you-sure.js:55

This is a fix for that use-case.