title: Allow form validation for Symfony 2 File constraint if empty
body: <p>I am trying to achieve the following based on this example:
  <a href="http://symfony.com/doc/current/cookbook/doctrine/file_uploads.html" rel="nofollow">http://symfony.com/doc/current/cookbook/doctrine/file_uploads.html</a>
  I have a virtual filed that is going to be used for uploading a file. Everything works perfect for the upload process. However, the file field in the form is always required probably because of</p>
  
  <pre><code>/**
   * @Assert\Image(maxSize="1000000")
   */
  public $image_virtual;
  </code></pre>
  
  <p>Is there any way to have a File or Image requirement only when the $image_virtual is filled? I want to be able to save the form even if the user doesn't input an Image</p>