Resolving default configuration error

So, when I was trying to build my jekyll site without setting no `sass` configuration in `_config.yml`, I was getting this error:

```shell
C:\wrks\git\kelvinst.github.io
λ jekyll build
Configuration file: C:/wrks/git/kelvinst.github.io/_config.yml
            Source: C:/wrks/git/kelvinst.github.io
       Destination: C:/wrks/git/kelvinst.github.io/_site
      Generating... error: undefined method `[]' for nil:NilClass. Use --trace to view backtrace
```

The error was pretty easy to solve (see the code).
Hope to see this in master soon.

Thanks.