[FIX] mass_mailing: properly handle multiple bounced_partner

It may be a recordset in case of partner duplicates. The variable
bounced_partner is simple search by email:

https://github.com/odoo/odoo/blob/72b5a17fc0e6f439d462647a7b55c9a26235a8df/addons/mail/models/mail_thread.py#L1349



closes odoo/odoo#69445

Signed-off-by: Ivan Yelizariev // IEL <yelizariev@users.noreply.github.com>