Fix error when JSON value is null

[cgarvis]

No description provided.

[cmontella]

Fixes: #478

[xymostech]

As mentioned in #478, this would mean that the null value doesn't get recorded in the record at all, so de-serializing then serializing again might lose keys. I would love this merged though because it's painful not being able to use APIs that have nulls in the responses.

[ibdknox]

Yeah, there's a follow up PR that needs to happen after this one that adds the sentinel value for json nulls.

[ibdknox]

Thanks!