1. Create an empty .ruby-version file
2. Run rv ruby list to find what the latest version is...

You just get the output × Empty input

Took me a couple minutes to remember the empty file as I'd set up shell integration in between so assumed that was the problem.

Perhaps the error message could be along the lines of "Empty Ruby version". I don't know how much context you have where the error is thrown but "Empty .ruby-version file in project folder" would be even clearer.