title: Does null=True imply default=None for models?
body: <p>When allowing database fields of a Django model to be NULL using <code>null=True</code>, is there default value guaranteed to be NULL by default? Or do I have to speficy this manually:</p>
  
  <pre><code>report = models.FileField(upload_to='reports', null=True, default=None)
                                                            ^^^^^^^^^^^^
  </code></pre>
  
  <p>I couldn't found anything about it in the documentation about <a href="https://docs.djangoproject.com/en/dev/ref/models/fields/#model-field-types" rel="noreferrer">models fields</a>.</p>