[FIX] stock_account: return product with PU equal to zero

To reproduce the issue:
(Need purchase,sale_management)
1. Create a product category PC:
    - Costing Method: FIFO
2. Create a product P
    - Type: Storable
    - Category: PC
3. Create a PO with 1 x P @ 10 + Receive P
4. Create a PO with 1 x P @ 0 + Receive P
5. Create a SO with 1 x P + Deliver P
6. Create a SO with 1 x P + Deliver P
7. Process a return for sold P at step 6
8. Open the valuation of the return

Error: The value is 10, should be 0

In `_get_price_unit`, `price_unit` is defined with the correct value
(i.e., `0`), but the if-condition of the `return` is not respected, so
another value is used

OPW-2735502

closes odoo/odoo#84274

Signed-off-by: William Henrotin (whe) <whe@odoo.com>