title: Symfony 3.0: Set Entities with fields that can have null
body: <p>I have an entity Users that has a field:</p>
  
  <pre><code>  /**
     * @ORM\Column(type="boolean", options={"default"=false})
     */
    private $activated;
  </code></pre>
  
  <p>On my database this is a not_null value. I want this (mapped) field to be able to accept null values.</p>