src: support domains with empty labels

Follow the spec of domainToASCII/domainToUnicode in whatwg, and
synchronise WPT url test data.

Refs: https://github.com/w3c/web-platform-tests/pull/5397
PR-URL: https://github.com/nodejs/node/pull/12707


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>