Default nullable fields to null in unique constraints checks

Author: browniebroke

rest_framework/serializers.py

@@ -1490,6 +1490,8 @@ def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs
                 default = timezone.now
             elif unique_constraint_field.has_default():
                 default = unique_constraint_field.default
+            elif unique_constraint_field.null:
+                default = None
             else:
                 default = empty