Optional int directive option incorrectly passed as empty string, not None

[zakhenry]

Describe the problem

I want to use the automatic dedent feature of literalinclude:

```{literalInclude} path/to/my/file
---
linenos: true
lines: 19-36
lineno-match: true
dedent:
---
```

With the above I get the following error:

WARNING: Directive 'literalInclude': Invalid option value: (option: 'dedent'; value: )
invalid literal for int() with base 10: ''

---
linenos: true
lines: 19-36
lineno-match: true
dedent: 
---

In the Sphinx definition the test is using None to test the feature, see https://github.com/sphinx-doc/sphinx/pull/8684/files

however in MyST-Parser, it coerces a None value into a string:

https://github.com/executablebooks/MyST-Parser/blob/master/myst_parser/parse_directives.py#L149-L150

Link to your repository or website

No response

Steps to reproduce

1. Using a Sphinx with MyST-Parser setup add the following code block

```{code-block} python
---
dedent: True
---
          print("Hello world")
```

2. See the following error in the console

 WARNING: Directive 'code-block': Invalid option value: (option: 'dedent'; value: )
invalid literal for int() with base 10: ''

---
dedent: True
---
          print("Hello world")

The version of Python you're using

Python 3.9.7

Your operating system

macOS 11.6

Versions of your packages

Sphinx v4.2.0

Additional context

No response

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[chrisjsewell]

thanks, yeh thats a pain, I will try to have a think about what the best way to handle this is