Fix empty file processing

The processing of an empty file returns the wrong source and breaks stylelint.

See [this issue](https://github.com/stylelint/stylelint/issues/7172) and this code:
```node
> require('postcss-scss').parse('').source
{
input: Input {
css: '',
hasBOM: false,
id: '',
[Symbol(fromOffsetCache)]: [ 0 ]
},
start: { column: 1, line: 1, offset: 0 },
end: { column: 1, line: 1, offset: 0 }
}

> require('postcss-sass').parse('').source
{
start: [ 0, 0 ],
end: [ 0, 0 ],
input: Input { css: '', hasBOM: false, id: '' }
}
```