[FIX] stock_account: price unit is zero

- Set a product P to average valuation, real time
- Create a PO for 1 unit @ 100, validate the picking
- Create a PO for 1 unit @ 0 (e.g. you receive a free product)

You cannot validate the picking because of the message 'The cost of P is
currently equal to 0...'

This error message is historical, to prevent users from an incorrect
configuration.

Since we still want to prevent misconfiguration, but support the
mentioned use case, we introduce an `ir._config_parameter` for people
who 'know what they are doing'.

opw-1962249

closes odoo/odoo#32550

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>