avoid implicit conversion of null in serializer (#609)

Author: Uzlopak

lib/serializer.js

@@ -54,7 +54,7 @@ module.exports = class Serializer {
     if (Number.isNaN(num)) {
       throw new Error(`The value "${i}" cannot be converted to a number.`)
     } else if (!Number.isFinite(num)) {
-      return null
+      return 'null'
     } else {
       return '' + num
     }