zeronull.Date

hi I have problem that `zeronull.Timestamp` is not omitted by json if the value is `NULL` string, the response still returning `{}`, also there is `pgtype.Date` but no `zeronull.Date`.

any help appreciated, thankyou.

Locate the existing zeronull.Timestamp implementation and pgtype.Date definition first. Compare how nullable values are represented and serialized, then identify the tests covering JSON omission; done means the requested zeronull.Date behavior and NULL-string omission are covered by tests.

Written by the indexing model from the issue text.