Comment on the end of selector causes warning

> WARNING on line 152 of .../incuna-sass/incuna-sass/_normalize.sass:
This selector doesn't have any properties and will not be rendered.

Sass no likey comment on end of selector. Doesn't seem to mind on the end of a property. But we don't really do comments on end of lines anymore, this one seems a bit overkill anyway.
I think this will have the same result as before: `button,` won't have any styles