title: Django: DateField "This field cannot be blank."
body: <p>I'm posting a rest request like this:</p>
  
  <pre><code>{title:some title, recurring:true, day:Wednesday, time:12:30, description:some text}
  </code></pre>
  
  <p>I am not passing the date because the event is recurring and the value should be empty.  The server response is:</p>
  
  <pre><code>{"date": ["This field cannot be blank."]}
  </code></pre>
  
  <p>Here is the relevant python code:</p>
  
  <pre><code>class Event(models.Model):
      title = models.CharField(max_length=200)
      recurring = models.BooleanField()
      day = models.CharField(max_length=20, blank=True)
      date = models.DateField(null=True)
      time = models.TimeField()
      description = models.CharField(max_length=500)
      venue = models.CharField(max_length=200, blank=True)
      venueAddress = models.CharField(max_length=200, blank=True)
      venueCity = models.CharField(max_length=200, blank=True)
  
  class EventSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Event
  
  class EventViewSet(viewsets.ModelViewSet):
      queryset = Event.objects.all()
      serializer_class = EventSerializer
  </code></pre>
  
  <p>I am not completely sure where the message is coming back from. Is my model defined correctly? Do I need extra work in my serializer?</p>