title: Doctrine ORM: NULL to DateTime conversion error
body: <p>I use PHP7, Symfony 2.8 andDoctrine ORM 2.5.
  I have an entity with a datetime field:</p>
  
  <pre><code> /** @ORM\Column(name="data_ordine", type="datetime", nullable=true) */
  private $dataOrdine;
  
  /**
   * @param mixed $dataOrdine
   */
  public function setDataOrdine($dataOrdine = null)
  {
      $this-&gt;dataOrdine = $dataOrdine;
  }
  
  /**
   * @return mixed
   */
  public function getDataOrdine()
  {
      return $this-&gt;dataOrdine;
  }
  </code></pre>
  
  <p>When i try to get the $dataOrdine field ($ordine->getDataOrdine()) of a persisted entity on MySQL database if the dataOrdine column is NULL i got: </p>
  
  <pre><code>object(DateTime)#551 (3) {
    ["date"]=&gt;
    string(27) "-0001-11-30 00:00:00.000000"
    ["timezone_type"]=&gt;
    int(3)
    ["timezone"]=&gt;
    string(13) "Europe/Berlin"
  }
  </code></pre>
  
  <p>instead of NULL</p>