[FIX] stock_account: valuation layer is zero when return dropshipping

To reproduce:
1. Create a Sales Order for a product whose product category is set to
   FIFO and automated. Use route "dropship".
2. Confirm the PO created.
3. Deliver the products (DS transfer)
4. Create the customer invoice
5. Return, for example, 1 unit of product
6. Add a credit note to the invoice for that 1 unit returned (reset to
   draft then change qty and post)
Issues:
The value on the valuation layers of the returned picking is 0, posted
entries for COGS and stock interim (delivered) account for credit note
is also 0.

Since #85751, When create valuation layer for return, we take all svls
of origin_returned_move_id into account to calculate the price unit.
However, then dropshiping, 2 svls are created for the original move, and
the sum of them is 0 since there is no impact of the stock when
dropshiping. So when return, the price unit will be calculated as 0.

To fix, when calculate price unit for return of dropshiping, we only
take non-negative svls into account. Note that we use non-negative ones
instead of positive ones because when subcontract dropshiping,
additional negative svls will be added for the cost of the components.

opw-3283436

closes odoo/odoo#123529

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>