If null or blank - don't save the nested object

rest_framework/serializers.py

@@ -891,6 +891,7 @@ class ModelSerializer(Serializer):
             # Nested relationships need to be saved before we can save the
             # parent instance.
             for field_name, sub_object in obj._nested_forward_relations.items():
+                if sub_object:
                     self.save_object(sub_object)
                 setattr(obj, field_name, sub_object)