[FIX] event: stop sending confirmation email as public user

Purpose
=======
Fix guest users receiving confirmation emails for events by "Public User".

Specifications
==============
Confirmation emails for events are sent by:

- Organizer (if set)
- Company email (if set)
- The users's email
- Odoo bot otherwise

Task-2657588

closes odoo/odoo#78849

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>