[FIX] mail: prevent useless write

In database with frontend and live chat there are lot of bad query error.
bad query: UPDATE "mail_channel_partner" SET "write_uid"=5,"write_date"=(now() at time zone 'UTC') WHERE id IN (328111)

closes odoo/odoo#70506

Error: Could not serialize access due to concurrent update
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>