[Cedar Doc Issue] Document behavior on `null` in entity json data

[john-h-kastner-aws]

After discussion on cedar-policy/rfcs#37 (rejected) we've decided that Cedar should not accept null json values anywhere in entity json data. We should update the documentation to specify this behavior.

[AMZ-brandon]

Hi John, just to be clear, this is the literal string null, not the absence of data in JSON as in [] or {}, correct?