title: Symfony2 form entity type with empty choices
body: <p>I have the form
  (buildForm method) : </p>
  
  <pre><code>    $builder-&gt;add('title')
          -&gt;add('orders', 'entity', array(
              'property' =&gt; 'name', 
              'choices' =&gt; array(), 
              'multiple' =&gt; true, 
              'required' =&gt; false
          )
       )
  </code></pre>
  
  <p>as you can see option 'choices' is an empty array, that`s because I have too large list of entities. So I load a few options for my entity select on client side  via ajax  (ajaxChosen),
  but after submitting my form I have a validation error on this field, because there are no such choices in array. How to solve this problem?</p>