feat: The tree can recognizes empty ul as a folder

[andybuibui]

🤔 这个变动的性质是？/ What is the nature of this change?

• [x] 新特性提交 / New feature
• [ ] bug 修复 / Fix bug
• [ ] 样式优化 / Style optimization
• [ ] 代码风格优化 / Code style optimization
• [ ] 性能优化 / Performance optimization
• [ ] 构建优化 / Build optimization
• [ ] 网站、文档、Demo 改进 / Website, documentation, demo improvements
• [ ] 重构代码或样式 / Refactor code or style
• [ ] 测试相关 / Test related
• [ ] 其他 / Other

🔗 相关 Issue / Related Issue

https://github.com/umijs/dumi/issues/2071

💡 需求背景和解决方案 / Background or solution

tree支持空 ul 也判定为文件夹

📝 更新日志 / Changelog

Language	Changelog
🇺🇸 English	The tree structure also treats empty ul as a folder
🇨🇳 Chinese	Tree支持空 ul 也判定为文件夹