title: how to filter a django model using IN for a list that includes None
body: <p>I have a view based on a model where the user can filter the items model based on the categories field using:</p>
  <pre><code>Items.objects.filter(categories__in=[‘A’, ’B’, ‘C’])
  </code></pre>
  <p>However in come cases the categories field is NULL, and the user may would like to see the list of NULL and A categories. So I have tried:</p>
  <pre><code>Items.objects.filter(categories__in=[None, ‘A’])
  </code></pre>
  <p>But this does not see to work and my result only shows for A,B and C</p>
  <p>This is, of course, a simplified view of the issue as the list is dynamic and there are so many other filters as well.</p>
  <p>But the principle is the same, how can include None in the list of IN operation.</p>