test: cover path empty string case

In path.toNamespacePath was a case when the path
was empty string and it wasn't covered in the tests.

I covered this case both in Windows and Unix environments.

PR-URL: https://github.com/nodejs/node/pull/24569


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>