fix(pathprefixer): correctly prefix root (./) directory

[Ruby184]

Proposed changes

Fix case when using PathPrefixer.prefixDirectoryPath for list with s3 and gcs.
When passing root directory ., / or ./ we should return empty prefix ''.
Added tests for failing cases before this change.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

Further comments

Prerequisite for correctly working list for s3 and gcs drivers (I will create PRs).

[thetutlage]

Cool, thanks!