Transaction with Cost with currency only not working in output to file

[hswong]

The following beancount transaction cannot be generated using the code directly as cost_to_str in position.py does not properly handle a Cost object with currency but no amount.

`

    posting1 = self._create_posting(transaction_date,
                                    account='Assets:Stock',
                                    units=BAmount(-1, 'AAPL'),
                                    cost=_Cost(None, currency, None, None),_
                                    price=None,
                                    flag=None,
                                    meta={})

    transaction = Transaction(meta={},
                                         date=transaction_date,
                                         flag="*",
                                         payee=None,
                                         narration='',
                                         tags=None,
                                         links=None,
                                         postings=[posting1, posting2])

`

Target output
2019-05-29 * "Sell a stock"
Assets:Stock -1 AAPL {USD}
Assets:Cash 150 USD

Actual output
2019-05-29 * "Sell a stock"
Assets:Stock -1 AAPL {}
Assets:Cash 150 USD

I had to patch position.py (in red) to be able to handle the above code and generate the target output above.