Use `null` in YAML instead of `nil`

I believe that in YAML, [`null` should be used rather than `nil`](http://yaml.org/type/null.html).

This is tricky because, in the Ruby source code, the values really are nil. That said, if somebody were to copy-paste the default config from the website, I believe that `null` is the most correct value to use.

If I am incorrect, or if this is a stylistic choice for Jekyll, please correct me and disregard this pull request.