Enable the EmptyRule SCSS Linter

Prevents empty SCSS rules from being included in the SCSS.

Also fixes the handful of lints caught by the linter.

Regarding the `.gh` rule in monokai.scss, which was added in [this commit](https://gitlab.com/gitlab-org/gitlab-ce/commit/bf079c24afb8ad2991a4eaf60a71a7bc45dd775d), it appears to be empty in [the original file as well](https://github.com/richleland/pygments-css/blob/master/monokai.css). So I think it's fine to delete the selector.

cc: @jschatz1 

See merge request !3769