buffer: stricter isEncoding

Due to a consolidation the isEncoding function got less strict in
version 5.x.x. This commit makes sure we do not return `true` for
empty strings.

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;