[FIX] base: merge contacts linked to by an o2m field with caps

Define a field on a model as:
- o2m to res.partner
- the field's column, hence its name, has capital letters in it
(studio does that)

create two objects of that class, each one linked to a different partner with the new o2m

merge the partners

Before this commit, the object linked to the second partner, was deleted
This was because merge partner sql requests did not quote the column name

After this commit, the second object still exists

This commit is tested in v12.0 with PR #30300 only. In v10.0 it is not testable as
the model concerned is in CRM, and that no new fields in business modules can be added in stable

OPW 1925060

closes odoo/odoo#30301