client.get_rates fails if store ID is not specified

Calling `get_rates` without store ID should work, as it does in [btcpay-python](https://github.com/btcpayserver/btcpay-python/) as well.

The problem is that node-btcpay literally passes `undefined` as store ID parameter if none is given, instead of just omitting the parameter in the URL.

Therefore, `client.get_rates(['BTC_USD'])` fails while it works to use `client.signed_get_request('/rates', {currencyPairs: 'BTC_USD'})` instead.

I think in general `signed_get_request` should omit `undefined` values.