[FIX] sale, sale_expense, sale_stock: make the False expense_policy selection behaves as 'no'

The sale.order model has an expense_policy selection field, with values in 'no',
'cost', 'sales_price'.
However that value can also be set to False.

The code that handles that field expected the value to always be set to a value.
We explicitly treat False as if it was 'no' (the default value for that field).

opw 1915508

closes odoo/odoo#30336