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