CSS file not generated when input is empty but when CSS should still be generated due to `prefix` being set

[kruncher]

If SCSS source file is empty and prefix is set then the CSS file should still be generated:

const scssConfig = {
  processor: () => postcss(postCssConfig),

  output: "./static-dev/css/main.css",
  outputStyle: "compressed",
  sourceMap: isPreview,
  failOnError: true,

  prefix: `@import "./frontend/styles/base.scss";`,

  includePaths: [
    "./node_modules/prismjs/themes",
  ],
};

[thgh]

It probably makes sense to always generate the CSS file for repeatability purposes. Could you make a PR?