title: Doctrine2 empty parameter on queryBuilder
body: <p>I am trying to check whether an user email is set or not. I am able to get the ones that are set to NULL but I am missing on the ones that have an empty string as the value. Here is my attempt:</p>
  
  <pre><code>$user = $this-&gt;createQueryBuilder('u')
          -&gt;where('(u.email IS NULL OR u.email = :empty)')
          -&gt;setParameter('empty', "''")
          -&gt;getQuery()-&gt;getResult()
  ;
  </code></pre>
  
  <p>I have no problem getting the NULL emails but I fail to get the empty string emails. Is there any way to accomplish this or is it not supported in DQL?</p>