buffer: fast-case for empty string in byteLength

When the string is empty, calling the binding is unnecessary and slow.

PR-URL: https://github.com/iojs/io.js/pull/1441


Reviewed-by: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Christian Tellnes <christian@tellnes.no>