When a user/partner is deleted, their messages lose their author.
Replying to those may result in a failure to fetch the mails from the
incomming mail server.

without this commit, partner_ids == [(4, False)] and this value "goes
down" all the way to the orm method "write" of the Many2many field.

resulting in a SQL query like :

INSERT INTO mail_message_res_partner_rel (mail_message_id, res_partner_id)
    (SELECT a, b FROM unnest(ARRAY[149855]) AS a, unnest(ARRAY[false]) AS b)
    EXCEPT (SELECT mail_message_id, res_partner_id FROM mail_message_res_partner_rel WHERE mail_message_id IN (149855))

which fails with the ProgrammingError : EXCEPT types boolean and integer
cannot be matched

opw-1921805

closes odoo/odoo#30528