title: How to allow empty values in symfony2 validators
body: <p>I would like to apply validators on a object properties only when value is not empty ie.</p>
  
  <p>Now standard symfony behavior:</p>
  
  <pre><code>class Entity
  {
      /**
       * @ORM\Column(type="string", nullable=true)
       * @Assert\Email()
       */
      protected $email;
      (...)
  }
  </code></pre>
  
  <p>that object will not pass validation if an email is null, or empty string, is there a way to tell validator to assert as a valid, an empty value, and validate only if field has data?</p>
  
  <p>PS I know that I can write callback validator, but writting callback for every field just to have "allowEmpty" feature isn't so nice.</p>