Fix bitstamp fee in case no transactions.

If an order is cancelled before there can be any transactions, the structure currently looks like this, which is a bit unexpected:

```
{'cost': None, 'currency': 'EUR'}
```

Full response:

```
{'id': '1777642340', 'datetime': None, 'timestamp': None, 'lastTradeTimestamp': None, 'status': 'Canceled', 'symbol': 'BTC/EUR', 'type': None, 'side': None, 'price': None, 'cost': None, 'amount': None, 'filled': 0.0, 'remaining': None, 'trades': [], 'fee': {'cost': None, 'currency': 'EUR'}, 'info': {'status': 'Canceled', 'id': 1777642340, 'transactions': []}}
```

Since the parsing logic is assuming that the result contains a transaction list with the fees, if there is such a list, but the list is empty, it  seem reasonable to assume that the fee then is 0.

[ci skip]
