Fix ProcessOptions interface

[ferreira-tb]

If I do something like:

const result = await postcss([plugin1, plugin2]).process(css)

This would correctly log the warning below:

Without `from` option PostCSS could generate wrong source map and will not find Browserslist config. Set it to CSS file path or to `undefined` to prevent this warning.

As explained, changing to process(css, { from: undefined }) prevent the warning. However, when exactOptionalPropertyTypes is set to true in the tsconfig.json it's not possible to do that at all, as undefined wouldn't be an allowed type.

TSConfig Reference: exactOptionalPropertyTypes

This PR only adds undefined to the type.

[ai]

LGTM. I will release it tomorrow.

[ai]

Thanks. Released at 8.4.32.