srv-rtlcss: Prevent from throwing

The problem is, when both rtlcss and autoprefixer
are enabled, it works fine with SCSS, but throws
with LESS.

When the code is too complicated,
the `result` object seems to have problem with
returning `result.css` member. When the code is
comparatively straight-forward, it throws a
slightly lenient error.

/cc @MohammadYounes