[FIX] stock_account: fix access error when computing average price

- Create a product with Category configured with:
  * Costing Method: First In First Out (FIFO)
  * Inventory Valuation: Automated
- Create a PO to buy 1 unit and receive the product
- Create a SO to sell 1 unit and deliver the product
- Return the product
- Re-deliver the product
- Create invoice for SO but don't post it
- Connect with an user with "Accounting: Accountant" rights only
- Post the invoice
An access error will be triggered when trying to access stock valuation layer.

opw-2623376

closes odoo/odoo#75507

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>