lib: use Object.create(null) directly

After V8 5.6, using Object.create(null) directly is now faster than
using a constructor for map-like objects.

PR-URL: https://github.com/nodejs/node/pull/11930
Refs: https://github.com/emberjs/ember.js/issues/15001
Refs: https://crrev.com/532c16eca071df3ec8eed394dcebb932ef584ee6


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>