Clarify handling of empty string in spec

[hkeide]

Hi, this PR introduces test cases for the empty string and empty suffix in the Go reference implementation: https://github.com/jetify-com/opensource/pull/352/files with a comment that it should also be considered for the spec. This is just meant to serve as a reminder to consider this.

I personally think that this could be a potential undetected edge case for interoperability between different implementations if it isn't present in the spec, and therefore I think it should be included.