Input `null` being deserialized as Java null value instead of `JsonValue.NULL`

[cowtowncoder]

(note: re-try fix for FasterXML/jackson-datatype-jsr353#16)

So, as per earlier take, JSON null should ideally map to JsonValue.NULL.

However! If nominal target type is NOT JsonValue but subtype like JsonObject, this is not possible and value MUST remain Java null.