Use silent comments to avoid outputting any code that's unnecessary

I think it's a bit more forgiving with weird edge cases.

```scss
  font-size: 3 + 0px; // 3px
  font-size: 3 * 1px; // 3px
  font-size: 3pt + 0px; // 3px
  font-size: 3pt * 1px; // 3pt*px isn't a valid CSS value.
```

Let me call @hugogiraudel to the rescue!