Allow no value

I'm trying to implement a string option that can have three states:
1. Not supplied
2. No value
3. Empty or value

I want to do the following:
`MyApp.exe` writes unformatted output to standard output
`MyApp.exe --json` writes JSON formatted output to standard output
`MyApp.exe --json file.json` writes JSON formatted output to `file.json`

Is this currently possible?

该 issue 未提及任何文件或测试。首先检查命令行选项解析的入口点，以及某个选项是否能够区分未提供、提供了但没有值、值为空或提供了显式值这几种情况。验证 issue 中的三种命令形式，并为每种预期输出和目标定义测试。

由索引模型根据 Issue 内容生成。