把处理空行的方式变得更优雅

backend/src/builtin-cli.cpp

@@ -117,6 +117,7 @@ void BookStoreMain(bool is_server, std::string config_dir) {
         std::getline(std::cin, cmd);
         auto ret = std::move(
             engine.Execute(cmd, session_map[session_token].login_stack));
+        if(ret.empty()) ret.push_back("[empty]");
         std::cout << session_token << ' ' << operation_token << " "
                   << ret.size() << std::endl;
         for (auto &line : ret) std::cout << line << std::endl;