Use a sentinel as default value for setting attribute only if changed

For new instances, we must set also the attribute with value `None` as they may
be tested later.

Closes #15060
(grafted from bfe3756af384f4fce94244761d5f5dd3f62d0ff6)

--HG--
branch : 8.0

modules/web_shop_shopify/common.py

@@ -188,6 +188,6 @@ class IdentifiersMixin(IdentifiersUpdateMixin):
 
 
 def setattr_changed(record, name, value):
-    if getattr(record, name, None) != value:
+    if getattr(record, name, object()) != value:
         setattr(record, name, value)
         return True