title: partial select doctrine query builder
body: <p>I'm trying to custom select use this code</p>
  
  <pre><code>$result = $qb-&gt;select('partial user.{id,name}')
                   -&gt;from('User', 'user')
                   -&gt;leftJoin('user.group', 'group')
                   -&gt;getQuery()
                   -&gt;getResult(); 
  </code></pre>
  
  <blockquote>
    <p>the result of this code in below</p>
  </blockquote>
  
  <pre><code>{
      "id": 10,
      "name": "admin",
      "description": null,
      "create_date": null,
      "modified_date": null,
  }
  </code></pre>
  
  <p>why returned field not selected with null value?!</p>