Fix field not null validate.

gm_serializer/fields/field.py

@@ -51,7 +51,14 @@ class MagicField(models.Field):
         :return:
         """
         if value is None:
-            return None
+            if self.null:
+                return value
+            else:
+                raise exceptions.ValidationError(
+                    self.error_messages['null'],
+                    code=['null'],
+                    params={'value': value, 'type': str(self.type)}
+                )
 
         if hasattr(value, 'id'):
             return self.type(value.id)