Activities and mailing trace are linked to documents through a model and
a many2one reference field. The latter one is required but is implemented like
an integer field, meaning writing 0 is actually a valid value for 'required'.
In this commit we add a constraint to ensure we never update activities with
a void res_id value. Same for mailing traces. This allows to remove the
required attribute on fields to avoid having redundant sql constraints.
Source: internal feedback about activities with 0 as res_id
Task-2694133
Part-of: odoo/odoo#81292
Thibault Delavallée
04ef6bee10