[FIX] mrp_account: AVCO product valuation with component cost 0

Steps to reproduce:
- Create a manufactured product P with AVCO valuation
- On the bom of the previous product add a component C with unit cost 5$
- Manufacture product P (Unit cost correct so far 5$)
- Change component cost to 0 and manufacture again
- Unit cost of P is still 5$ were it should be 2.5$

Bug:
since the move unit price is 0 the standard price of the product P is
used insted for the valuation of the newly created qty

Fix:
force unit price to 0 in the case of manufacturing

opw-3374462

closes odoo/odoo#130592

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