[FIX] website_event_track: fix new track notifications by email

Bug
===
1. Create an event which allows track proposal
2. Follow it and subscribe to "New Track"
3. Log in in incognito and submit a proposal

The email is not sent, because it's sent as the public user, which has
no email address set. And so it the 2 system parameters
<mail.catchall.domain> and <mail.default.from> are not set, we can not
know which email address used to send the email.

Note that this bug also occurs if you create a track with a user without
an email address set.

Task 2510181

closes odoo/odoo#69890

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