Not that concerning, I'll even argue that it's not concerning at all :P

Sitemaps allow webmasters to index more paths of their website to try to be more relevant but: the most important path that search engines have to index about SN is /items, and they do a pretty good job at it.

Robots.txt instead does the opposite. It allows webmasters to hide paths from indexing, but there isn't a single path that we would wanna hide, so it would be an empty file.

These files are optional, needed in websites that uses some kind of CMS that has weird, intricate paths, exposes configuration files, and so on — we don't do that here.

tldr: don't worry about it! SN is already well indexed ^^ But I welcome any discussion on it since I can also be wrong