title: Django: filtering queryset by 'field__isnull=True' or 'field=None'?
body: <p>I have to filter a queryset by a dynamic value (which can be None): may I simply write:</p>
  
  <pre><code>filtered_queryset = queryset.filter(field=value)
  </code></pre>
  
  <p>or shall I check for None:</p>
  
  <pre><code>if value is None:
      filtered_queryset = queryset.filter(field__isnull=True)
  else:
      filtered_queryset = queryset.filter(field=value)
  </code></pre>
  
  <p>Does the behaviour depend on the particular DBMS?</p>