Frontmatter defaults in _config.yml are not correctly applying to pagination pages

[UlyssesZh]

Frontmatter defaults in _config.yml are not applying to pagination pages. This is because frontmatter defaults rely on Convertible#relative_path:

https://github.com/jekyll/jekyll/blob/d86ba153fe012fbf4c62dc4af9dd73df1f1cfd58/lib/jekyll/convertible.rb#L249-L251

Then, PaginationPage inherits #relative_path from Page, where this method is dependent on @dir:

https://github.com/jekyll/jekyll/blob/d86ba153fe012fbf4c62dc4af9dd73df1f1cfd58/lib/jekyll/page.rb#L143-L146

However, PaginationPage objects have their @dir to be nil, so the path information is wrong when Convertible#to_liquid looks at their frontmatter defaults.