class Product(models.Model):
    title = CharField(max_length=512)
    attributes = HStoreField(null=True, blank=True)

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ('title', 'attributes')

p = Product(title="foobar", attributes={'first': 'value', 'second': None})
p.save()     # Save success. No error here!


# Now, Let's try with Serializer
data = {"title": "foobar", "attributes": {"first": "value", "second": None}}
q = ProductSerializer(data=data)
q.is_valid()
#> False
q.errors
#> {'attributes': ['This field may not be null.']}