[FIX] stock_account: fix valuate empty stock.

To reproduce, product A in AVCO:
- Buy 2 product A at 1.00 $
- Buy 1 product A at 1.01 $
- Sell 3 product A.
- The stock valuation won't be correct: still remain 0.01 in
stock without quantity and the svl related to the sell
have a value of 3.00 (instead of 3.01)

To fix:
In case of AVCO, add the rounding error value to the out
stock move layer.

closes odoo/odoo#46850

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>