fix: Do not error if a directive has no options or body

[jpmckinney]

It is allowed to do, for example:

```{directive} required argument
```

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[hukkin]

Good catch!

I suggest the following:

1. Move the handling from format_directive_content to parse_opts_and_content (make it return None)
2. Add a test case to tests/data/fixtures.md. E.g.

   MyST directive, no opts or content
   .
   ``` {some-directive} args
   ```
   .
   ```{some-directive} args
   ```
   .

[jpmckinney]

Thanks for the review. Done now!

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