buffer: stricter isEncoding

Due to code consolidation in https://github.com/nodejs/node/pull/7207
the isEncoding function got less strict. This commit makes sure
isEncoding returns false for empty strings as before the consolidation.

PR-URL: https://github.com/nodejs/node/pull/18790
Refs: https://github.com/nodejs/node/pull/7207


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>