maintenance: enable `strictNullChecks` option

[JounQin]

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Initial checklist

• [X] I read the support docs
• [X] I read the contributing guide
• [X] I agree to follow the code of conduct
• [X] I searched issues and couldn’t find anything (or linked relevant results below)

Problem

https://github.com/mdx-js/eslint-mdx/pull/521#discussion_r1451650183

Solution

As title

Alternatives

N/A