pre-compute empty string (#573)

Author: Uzlopak

lib/serializer.js

@@ -90,7 +90,7 @@ module.exports = class Serializer {
     if (str instanceof Date) {
       return quotes + str.toISOString() + quotes
     } else if (str === null) {
-      return quotes + quotes
+      return '""'
     } else if (str instanceof RegExp) {
       str = str.source
     } else if (typeof str !== 'string') {