title: select2 - how to allow a null value
body: <p>In a form there are multiple select2 elements.
  In a particular select2 element I set some options with a not null string 'text' and null 'value'.
  The submit form seems to behave like this:</p>
  
  <ul>
  <li>if 'value' is not null -> 'value' is the submitted item on 'submit' action</li>
  <li>if 'value' is null -> 'text' is the submitted item on 'submit' action</li>
  </ul>
  
  <p>This is not the behavior I'm looking for.
  How can I submit 'value' when 'value' is null? I just want to submit null! I can't find anything like that in the select2 documentation.</p>