One2One field should return None when field has no value

review12691002
issue4335
(grafted from edef4acbbdd8208c3cb6e2b62017a18ccff38c64)

--HG--
branch : 2.6
extra : source : edef4acbbdd8208c3cb6e2b62017a18ccff38c64
Add Id into pyson.CONTEXT

issue4352
review14701002
(grafted from 86f7d18d5f93b52c4774f90fb69cf3b3d509ce82)

--HG--
branch : 2.6
extra : source : 86f7d18d5f93b52c4774f90fb69cf3b3d509ce82