Fix condition in _on_change_set method

If _on_change_set is called with a None value for a XXX2Many field, the test
designed to check for creations / updates raises an Exception because of
missing parenthesis.

issue4284
review10591002
(grafted from f190c758b197b500a5a101e68fa2d28f01a78724)

--HG--
branch : 2.6
extra : source : f190c758b197b500a5a101e68fa2d28f01a78724