src: reword command and add ternary

Make comment clear that Undefined() is returned for legacy
compatibility. This will change in the future as a semver-major change,
but to be able to port this to previous releases it needs to stay as is.

PR-URL: https://github.com/nodejs/node/pull/5756


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>