add db/string test

# 变更

增加DB包string.go单元测试

# 检查清单

* [ ] 是否关联Jira Issue
* [ ] 是否有单元测试
* [ ] 是否完成本地测试
* [ ] 是否更新文档

# Reviewers

* [x] reviewed @ryq
* [ ] reviewed @jiaolong
* [ ] reviewed @lza
* [ ] reviewed @whj1
* [ ] reviewed @zhs1
* [ ] reviewed @lgx

See merge request !46