private <T> Mono<T> emptyResponse() {
        return EMPTY_RESPONSE;
    }
@author James Chen
@implNote Don't use the Elasticsearch official implementation because:
          <p>
          1. Its jar (elasticsearch-java:8.13.0 is 11.2 MB) is unnecessarily large for our use
          cases.
          <p>
          2. Our implementation is more concise and efficient.
turms-service/src/main/java/im/turms/service/storage/elasticsearch/ElasticsearchClient.java