Treat JSON null as empty value

I need to have JSON `null` literal values normalised to empty strings.

I realise the JSON specification also allows `true` and `false`; thus this specific implementation for `null` _MAY_ set a precedent for the other two. Should we normalise those as strings `"true"` or `"false"`. If so, why not set `null` to `"null"` as well?

Anyway, I just want to bring up the discussion based on my personal needs. Ultimately, it all depends on how much opinionated you want PapaParse to be. Feel free to close this PR without merging in case you don't agree with the solution.

Thanks for your great work here! :+1: