[FIX] point_of_sale, stock: do not crash if no email set

- Create a POS user for which the corresponding partner has no email
  address.
- Connect to the POS with the created user
- Sell a product which creates a picking (e.g. Customizable Desk)

The picking is created in draft and not validated.

This is due to:

https://github.com/odoo/odoo/blob/6435b27c35faeafca452d26fa7fd50a7fe964029/addons/mail/models/mail_thread.py#L2199



We disable tracking and post the message as `sudo()`.

opw-2091808

closes odoo/odoo#39423

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>