The string "null" is formatted as null

The string "null" is formatted as `null` itself, causing confusion.

Screenshot: https://imgur.com/yx81O8Z

This is on the Chrome extension as installed from the Chrome Web Store at https://chrome.google.com/webstore/detail/immutablejs-object-format/hgldghadipiblonfkkicmgcbbijnpeog?hl=en

This appears to be happening because of this line: https://github.com/andrewdavey/immutable-devtools/blob/master/src/createFormatters.js#L37

It can probably just be removed because actual nulls are formatted correctly by the fallthrough case that formats all objects.