@clayui/css: Mixins `clay-button-variant` remove unnecessary setter()

The `setter()` function was used to set `null` parameters to empty maps so the Sass function `map-get` wouldn't throw the error:

```
Error: argument `$map` of `map-get($map, $key)` must be a map
```

We now check to see if the parameter is of type `map` in the `clay-css` mixin and no longer need to do this.