[FIX] mail: Harmonize _mail_find_partner_from_emails returned values

Purpose
=======

Before this commit, the method _mail_find_partner_from_emails could
return a list mixing partner ids and partner recordset, that could
lead to issue when trying to read the different elements.

With this commit, we ensure that we return a list of records only.

closes odoo/odoo#44588

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>