* fix: price list with 0 value are ignored
Steps to reproduce:
1. Create 2 item price for two different supplier. One of them should be
zero.
2. Create PO
3. Add supplier with non-zero price and add item.
4. change supplier. Price won't change. If price was non-zero it
would've changed.
Root cause: falsiness check instead of null value check is used for
checking if price list value exists. 0 is evaluated as false.
* refactor: make get_price_list_rate function pure
16d4de5130