code: public static Single<Boolean> isEmpty(CacheConfigBean cacheConfigBean, String cacheKey) {
          return length(cacheConfigBean, cacheKey).map(length -> length == 0);
      }
docs: check whether the cached list is empty or not.
  
  @param cacheConfigBean the data source of the cache
  @param cacheKey        the key for the cached list
  @return true is the cached list is empty or does not exists, other wise false.