buffer: stricter isEncoding

Due to code consolidation in nodejs#7207
the isEncoding function got less strict. This commit makes sure
isEncoding returns false for empty strings as before the consolidation.

PR-URL: nodejs#18790
Refs: nodejs#7207
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/buffer.js

@@ -427,7 +427,7 @@ Buffer.compare = function compare(a, b) {
 
 
 Buffer.isEncoding = function isEncoding(encoding) {
-  return typeof encoding === 'string' &&
+  return typeof encoding === 'string' && encoding.length !== 0 &&
          normalizeEncoding(encoding) !== undefined;
 };
 Buffer[kIsEncodingSymbol] = Buffer.isEncoding;