buffer: fast-case for empty string in byteLength

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

PR-URL: #1441
Reviewed-by: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Christian Tellnes <christian@tellnes.no>

Author: JacksonTian

lib/buffer.js

@@ -276,6 +276,9 @@ function byteLength(string, encoding) {
   if (typeof(string) !== 'string')
     string = String(string);
 
+  if (string.length === 0)
+    return 0;
+
   switch (encoding) {
     case 'ascii':
     case 'binary':