Original PR description

**Steps to reproduce:**
- Install Accounting, Sales and l10n_pe_edi
- Switch to a Peruvian company (e.g. PE Company)
- Create a SO for a Peruvian customer (e.g. Comercial Constructora)
- Confirm the SO
- Create a down payment
- Confirm the down payment
- Send it to Peru UBL 2.1

**Issue:**
The Electronic Service Operator (OSE) responds with an error because an item is declared without unit of measure.

**Cause:**
From saas-17.1, there is no product associated to a down payment line anymore.

**Solution:**
For down payment line, use the code (i.e. NIU) of the default unit of measure (i.e. Units), which was the default UoM of the down payment product before saas-17.1.

opw-4057252