test: increase coverage of Buffer.transcode

Adds test for transcoding an empty buffer.

PR-URL: https://github.com/nodejs/node/pull/10437


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>