[FIX] stock_account: don't update inventory valuation for negative quantity

Expected Behaviour

When updating the cost of a product with a negative quantity and then
making an inventory adjustment, the final value should be corrected according
to the current cost of the product

Observed Behaviour

In the case we had a negative quantity when changing the cost, the compensation
layer will be ignored when doing an inventory adjustment afterward, leading to
an incorrect total value in the inventory valuation

Reproducibility

This bug can be reproduced following these steps:
- Create Storable product with costing method = AVCO Automated, Update cost to 100$
- Sell 10 units of this product (Inventory valuation at -1000 $ for -10 units)
- Update cost of the product to 10 $ (Inventory valuation at -100 $ for -10 units)
- Create an inventory adjustment to set the quantity of the product to 0.
==> Inventory valuation is a 900 $ for 0 unit of our product

Related Issues/PR

- opw-2635692

closes odoo/odoo#85894

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