Incorrectly imports .CSS files that are already minified

[samhh]

As mentioned on our Impero website issue:

If you import a non-minified .CSS/.SSS file, no problem.
If you import a minified .SSS file, no problem.

If however you import a minified .CSS file, Sourdough will incorrectly parse it and remove all curly braces.

This was through gulp-sourdough but the problem probably lies here.