[FIX] hr: singleton error on writing user to employee

before this commit, on writing user_id to hr.employee
on multiple record in raisng single ton error.

after this commit, no error wont be raised on the
same.

closes odoo/odoo#130019

X-original-commit: d4614015
Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>
Signed-off-by: Kevin Baptiste <kba@odoo.com>