Shows arrow when multiple non-nesting categories are set

When categories are set like this:
```
categories:
- [Featured]
- [Tutorial]
```
Landscape theme shows arrows between categories, although they are not nested:

Discussed [here](https://github.com/hexojs/hexo-migrator-wordpress/issues/36#issuecomment-664199660) and [here](https://github.com/hexojs/hexo-migrator-wordpress/issues/54#issuecomment-664005653)

> The theme displays it as nested, but if you check the `Tutorial` link, it should be `http://yourhexo.com/categories/tutorial`, not `http://yourhexo.com/categories/featured/tutorial`. Perhaps a theme couldn't differentiate whether a category is nested or not, but that's a separate issue.

Reproduce the issue with the two non-nesting categories shown in the report and inspect the Landscape theme's category navigation rendering. Review the linked discussions for the distinction between nested and separate categories. Done means arrows appear only for actual nesting, while the Tutorial link remains at /categories/tutorial.

Written by the indexing model from the issue text.