Don't throw on empty rates

When getting rates with the `getLowestHotelPrices`
method, we might not get any rates back.
However, we shouldm't assume that this
means our `hotelCode` was wrong, It could
be the hotel isn't open during that period.
Instead, we should rely on an empty `currecyCode`.

index.js

@@ -175,7 +175,7 @@ export default class Trippe {
       .then(hotel => {
         const { currencyCode, rates } = hotel
 
-        if (rates.length === 0) {
+        if (currencyCode === '') {
           throw new Error('Unknown or invalid hotelCode')
         } else {
           const ratesCombined = rates.flatMap(rate => rate.windows)