[FIX] mail: add check valid email address

Purpose
=======
The method find_or_create of res.partner overwritten in mail does not
take into account the parameter assert_valid_email. This commit adds a
simple check and test.

Task-2799577

closes odoo/odoo#86943

X-original-commit: 7a07b365
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>