Ignore empty version directories of dotnet

[Herrmel]

#3261

Problem

Dotnet core assemblies can not be resolved if empty folders of previously installed versions exist.

Solution

• Any comments on the approach taken, its consistency with surrounding code, etc.
  Ignore empty Directories
• Which part of this PR is most in need of attention/improvement?
  I changed the result of ConvertToVersion only check for empty directories where the directory is considered a fitting version. I hope that is fine.
  In my case the version directories where completly empty but maybe it would make sense to look for at least one file that does have .dll or .exe extension?
• At least one test covering the code changed
  I'm not sure how to test things that are environment dependend.

[siegfriedpammer]

Thank you for your contribution!