Fixes empty z-index property to be 'auto'.

Fixes a bug that causes a z-index property to have no value. Rather than using 'null' (which results in no value for the property), 'auto' is used instead.

This bug is a problem if you use a build process like Uncss to trim unused CSS. The build will fail as the empty z-index property isn't valid.
Minor typo in comments.

Minor, but it bugged me.
Merge pull request #5773 from jonathanmelville/master

Sass compile of _reveal.scss results in empty z-index property