*change default value from none to undefined if field does not exist

schematics/models.py

@@ -75,8 +75,8 @@ class Model(metaclass=ModelMeta):
         filter_func = get_role_func(self.__class__, context)
 
         for field, f_obj in self.fields.items():
-            f_value = data.pop(field, None)
-            if f_value is None:
+            f_value = data.pop(field, Undefined)
+            if f_value is Undefined:
                 f_value = f_obj.default
                 if f_value == Undefined:
                     continue