[FIX] hr_expense: error on duplication with uom

Step to reproduce:
- Duplicate expense with a uom != of Unit

Current behaviour :
- Error telling that the unit is not correctly chosen

Behaviour After PR:
- The uom is copied on duplication and no error

opw-2714136

closes odoo/odoo#81638

Signed-off-by: Damhaut Florian (flda) <flda@odoo.com>