[FIX] get interest for currency without interest
[FIX] get interest for currency without interest

- `get_interest_at` for a currency without interest would not hit the intended check since the function was used as variable instead of calling the method
ripple/ripple-lib,darkdarkdragon/ripple-lib,darkdarkdragon/ripple-lib,ripple/ripple-lib,wilsonianb/ripple-lib,wilsonianb/ripple-lib,wilsonianb/ripple-lib,ripple/ripple-lib,ripple/ripple-lib,darkdarkdragon/ripple-lib