[FIX] mail: defensively evaluate 'partner_to' on mail templates

MailTemplate model has a 'partner_to' field is dynamically rendered to contain
partners being recipients. After rendering it should hold a comma-separated
list of partner IDs.

However as we are unsure it was correctly written better be defensive. We now
check that we can effectively transform items to IDs using 'isdigit()'.

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#74474