buffer: improve Buffer.byteLength(string, encoding)

When string is empty, it will running into binding also.
It make the performance is wasted.

lib/buffer.js

@@ -276,23 +276,27 @@ function byteLength(string, encoding) {
   if (typeof(string) !== 'string')
     string = String(string);
 
-  switch (encoding) {
-    case 'ascii':
-    case 'binary':
-    case 'raw':
-      return string.length;
+  if (string.length > 0) {
+    switch (encoding) {
+      case 'ascii':
+      case 'binary':
+      case 'raw':
+        return string.length;
 
-    case 'ucs2':
-    case 'ucs-2':
-    case 'utf16le':
-    case 'utf-16le':
-      return string.length * 2;
+      case 'ucs2':
+      case 'ucs-2':
+      case 'utf16le':
+      case 'utf-16le':
+        return string.length * 2;
 
-    case 'hex':
-      return string.length >>> 1;
-  }
+      case 'hex':
+        return string.length >>> 1;
+    }
 
-  return binding.byteLength(string, encoding);
+    return binding.byteLength(string, encoding);
+  } else {
+    return 0;
+  }
 }
 
 Buffer.byteLength = byteLength;