Set relation_field of one2many only when exists

The client use this value if it exists for string concatenation, so having None
fails.
(grafted from 070f4178a35b304b9e8f3e5e7f0a103621447dad)

--HG--
branch : 3.2
extra : source : 070f4178a35b304b9e8f3e5e7f0a103621447dad