trade: review Valuation Method field on Inventory Report

- add an empty item to the list, so that user have to choose something. The empty item was missing here:

![image](/uploads/5be3e9d5acf1233270516c7463dbfdd4/image.png)

If there's an empty item and we force user to choose, there will not be the problem that user can forget to select the good value and mistakenly generates for the Fifo because it's the first value.

- translate values

See merge request nexedi/erp5!1204