bug: nav title cannot work in singe 2-level directory

[PeachScript]

Version

2.2.17

OS Version

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Node.js Version

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Browser Version

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Link to minimal reproduction

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Steps to reproduce

1. 在 examples/normal 下创建 docs/a/b/c.md
2. 给 c.md 配置 nav.title，检查效果
3. 给 c.md 配置 nav.second.title，检查效果

What is expected?

nav.title 生效 + nav.second.title 不生效，因为此时只有一组二级文件夹，尚未满足生成二级导航的条件

What is actually happening?

nav.title 不生效 + nav.second.title 生效

Any additional comments? (optional)

No response