Initial checklist

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

Problem

mdx-js/mdx#2136

Solution

Throw here when neither plugins nor settings is defined.

Alternatives

Keep as is.

I don’t imagine most folks want empty presets being silently ignored.

For unified-engine, we treat config files as presets as well (though importing plugins-as-string). There might be a tiny use case there for empty config files, as they could “restore-to-defaults” a folder, if a higher folder has a config file that prescribes complex things.
This would break that use case.
But I dunno, I don’t think people actually do this?