Fix option validation of null

[davidtheclark]

Fixes #1964 and adds several tests to prevent future regressions.

Most of the diff in validateOptions.js is the result of pulling an unnecessarily nested function to the top level. The key difference is that I added this line (47):

if (actual === null || _.isEqual(actual, [null])) { return }

@jeddy3: If you think this looks good let's release the patch right away.

[jeddy3]

@jeddy3: If you think this looks good let's release the patch right away.

Agreed. I'll merge and release now.

Thanks for doing the fix and for adding the tests!

I guess none of the core team is using null in their configs for the whitelist and blacklist rules. I suspect the regression only affected those who used the copy-pasta example config and used null to turn off the rules they didn't use (rather than delete them).