process.env 赋值的属性会隐式转化为 string 类型

[nmsn]

来源：https://nodejs.org/dist/latest-v8.x/docs/api/process.html#process_process_env

process.env.test = null;
console.log(process.env.test);
// => 'null'
process.env.test = undefined;
console.log(process.env.test);
// => 'undefined'