Convert node.meta to undefined if null

[hugmanrique]

@mattdamon108 described the following warning being printed into the browser's console:

index.js:2178 Warning: React does not recognize the `metaString` prop on a DOM element. If you intentionally want it to appear in the DOM as a custom attribute, spell it as lowercase `metastring` instead. If you accidentally passed it from a parent component, remove it from the DOM element.
    in code (created by MDXTag)
    in MDXTag (created by Context.Consumer)
    in Unknown (at this-is-the-slug.mdx:17)
    in pre (created by MDXTag)
    in MDXTag (created by Context.Consumer)
    in Unknown (at this-is-the-slug.mdx:17)
    in main (at Layout.js:11)
    in div (at Layout.js:6)
    in Layout (at BlogLayout.js:4)
    in BlogLayout (at this-is-the-slug.mdx:14)
    in div (created by MDXTag)
    in MDXTag (created by Context.Consumer)
    in Unknown (at this-is-the-slug.mdx:12)
    in Unknown (at _app.js:15)
    in Container (at _app.js:14)
    in MyApp

This issue was supposed to have been fixed on #294, but a change on c26a03d#diff-fd729e637d80469ad8fcafd812c97b44 changed the value of props.metastring to node.meta (a value created by mdast), which, by looking at the specification, will be null instead of undefined if no meta string is present after the language.

mdast Code specification

I haven't checked the React source, but it is probably complaining because it doesn't know how to convert a null prop into a DOM attribute value.

Unrelated note: I couldn't get the previous commits from the last PR I made to go away, feel free to squash all the commits before merging.

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[mununki]

@hugmanrique Thanks for your fix. I await this PR is merged soon.

[johno]

Thank you @hugmanrique!