Credit Control Dunning Fees: improve handling of currencies

Remove required on the currency and fallback to company's currency on the
policy level if it is empty.

Also, it fixes a bug when a credit line had no currency and the level had
the company's currency. As False is different than any currency, it entered
in res_currency.compute() with a False currency. It must just fallback on
the company's currency if the credit line's currency is empty.