fix: return empty if the key-value is empty value

PR-URL: https://github.com/koajs/locales/pull/17
Reviewed-By: fengmk2 <fengmk2@gmail.com>
Reviewed-By: dead-horse <heyiyu.deadhorse@gmail.com>

test/locales/zh-CN.js

@@ -2,6 +2,7 @@
 
 module.exports = {
   Email: '邮箱',
+  emptyValue: '',
   'Hello %s, how are you today?': '%s，今天过得如何？',
   model: {
     user: {

test/locales/zh_TW.json

@@ -1,5 +1,6 @@
 {
   "Email": "郵箱",
+  "emptyValue": "",
   "Hello %s, how are you today?": "%s，今天過得如何？",
   "model": {
     "user": {