Throw an exception on a never null method when result set is empty

Author: vincenthsin

src/main/java/javax/money/spi/MonetaryCurrenciesSingletonSpi.java

@@ -169,7 +169,7 @@ default Set<CurrencyUnit> getCurrencies(String... providers) {
     default CurrencyUnit getCurrency(CurrencyQuery query) {
         Set<CurrencyUnit> currencies = getCurrencies(query);
         if (currencies.isEmpty()) {
-            return null;
+            throw new MonetaryException("CurrencyUnit not found: " + query);
         }
         if (currencies.size() == 1) {
             return currencies.iterator().next();