[FIX] point_of_sale: User associated with non-existing Employee

* Install pos_hr
* Create new user [DEMO] (change password and save the user id)
* In POS session settings, uncheck "Login with Employees"
* Log in with [DEMO]
* Start a new POS session
* From the Employee page delete the record matching [DEMO] user id
* Process a transaction from the POS

Several error message will popup. This is due to
https://github.com/odoo/odoo/commit/26274dbb9720743461be2a70de02bf4c1ea377f4


which introduced a faulty matching between the id of user who start the pos
and an employee with the same id.
Assigning the single properties as before the change fix the issue

opw-2087591

closes odoo/odoo#39028

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>