Breaks when the source file only contains @at rules.

Hello!

When processing a source file which only contains at-rules, the plugin breaks.

From my error code and brief skim, it seems to occur from Line 123 of `index.js`.

The error reported reads as: `Cannot read properties of undefined (reading 'source')`.

The problem file in question is a file from open-props: [`open-props/media.css`](https://github.com/argyleink/open-props/blob/main/src/props.media.css).

I will do a workaround and create a pull request once I get home.

Thanks