code: public static Optional<Long> getDurationOptional(Config config, String path,
              TimeUnit timeUnit) {
          return Optional.ofNullable(getDuration(config, path, timeUnit));
      }
docs: Get a configuration as duration (parses special strings like "10s"). Return {@code null}
  if missing, wrong type or bad value.
  
  @param config
  @param path
  @param timeUnit
  @return