fix panic caused by string is nil #55

	#-
	# #%L
	# OBKV Table Client Framework
	# %%
	# Copyright (C) 2023 OceanBase
	# %%
	# OBKV Table Client Framework is licensed under Mulan PSL v2.
	# You can use this software according to the terms and conditions of the Mulan PSL v2.
	# You may obtain a copy of Mulan PSL v2 at:
	# http://license.coscl.org.cn/MulanPSL2
	# THIS SOFTWARE IS PROVIDED ON AN "AS IS" BASIS, WITHOUT WARRANTIES OF ANY KIND,
	# EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO NON-INFRINGEMENT,
	# MERCHANTABILITY OR FIT FOR A PARTICULAR PURPOSE.
	# See the Mulan PSL v2 for more details.
	# #L%

	name: CI

	on:
	workflow_dispatch:
	push:
	branches:
	- main
	pull_request:
	branches:
	- main

	jobs:
	build:
	name: Build
	runs-on: ubuntu-latest
	steps:
	- name: Check out code into the Go module directory
	uses: actions/checkout@v2

	- name: Set up Go
	uses: actions/setup-go@v3
	with:
	go-version-file: 'go.mod'

	- name: Cache Go modules
	uses: actions/cache@v2
	with:
	path: ~/go/pkg/mod
	key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
	restore-keys: \|
	${{ runner.os }}-go-

	- name: Install dependencies
	run: go mod download

	- name: Build
	run: \|
	go list ./... \| grep -v /example/ \| grep -v /test/ \| xargs -L 1 go build -v

	- name: Run unit tests
	run: \|
	go test -v $(go list ./... \| grep -v /test/ \| grep -v /example/)

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