Ignore null properties inside JSON

[ChloePont]

The following event

{
    "id": "aaaa-bbbb",
    "json": "{\"user_id\":null, \"received_at\":\"2021-03-31T10:03:51.966Z\"}"
}

should return the following schema:

{
    "id": STRING,
    "json": 
        {"received_at": STRING}
}