code: public Set<String> toSet(Set<String> defaultValue) {
          List<String> list = toList();
          if (list.isEmpty()) {
              return defaultValue;
          }
          return Collections.unmodifiableSet(new HashSet<>(list));
      }
docs: Converts a string value(s) to a Set. default value specified would be returned
  if value is empty or null
  
  @param defaultValue the value to be returned by default, when value(s) is not present
  @return an Immutable Set