[FIX] stock_account: Average Cost

- Create a product where the associated product category has Costing
  Method "Average Cost (AVCO)"
- Create a PO. Order:
  2 units at 40
  4 units at 35
- Receive the picking

The cost of the product is set at 35 (or 40, depending on the order of
the lines in the PO).

Since we are in a situation where the available quantity is zero, we
enter in the `if float_is_zero...` condition, where the `qty_done` is
not set.

We should set it in all cases.

opw-1907328

closes odoo/odoo#30422