title: Allow null in foreign key to user. Django
body: <p>I have this model</p>
  
  <pre><code>class Vacancy(models.Model):
      user = models.ForeignKey(User, null=True, blank=True, default = None)
      name = models.CharField(max_length=64)
  </code></pre>
  
  <p>When in admin i try to creat a vacancy without a user. And it throws an error " club_vacancy.user_id may not be NULL".
  Am i doing something wrong?</p>