Missing JsonNode field deserialized to null instead of NullNode

[the8tre]

Describe the bug
There's a behavior change between 2.12.6 and 2.13.0.
Until then a missing JsonNode field was deserialized as a NullNode and it now is deserialized to Java null.

Version information
Behavior change occurs since 2.13.0

To Reproduce

    public static class MyObject {
        public JsonNode jsonNode;
        @JsonCreator
        public MyObject(@JsonProperty("jsonNode") JsonNode jsonNode) {
            this.jsonNode = jsonNode;
        }
    }
    
    @Test
    void nullNodeTest() throws JsonMappingException, JsonProcessingException {
        String json = "{}";
        ObjectMapper mapper = new ObjectMapper();
        MyObject object = mapper.readValue(json, MyObject.class);
        assertNotNull(object.jsonNode);
        assertInstanceOf(NullNode.class, object.jsonNode);
    }

With 2.12.6this test passes successfully.
Since 2.13.0it fails as jsonNode is deserialized to null.

Expected behavior
After I've read a lot of issues related to null and NullNode and I can't tell for sure what should be the expected behavior.
But for sure that change can break some code relying on that.

Additional context
The change introduced in 2.13.0 to fix #3214 seems to be somewhat linked to this behavior change.

[wix-andriusb]

@cowtowncoder Is this the new expected behavior?
Or can we expected this to be reverted?

[wix-andriusb]

looks like this is intended: #3214 (comment)

[cowtowncoder]

Correct: as per #3214 this works as intended and is the new expected behavior.

If coercion from null to NullNode is preferred, it should be possible to add "null conversion" logic (either with @JsonSetter, or ObjectMappers configuration) using Nulls.AS_EMPTY for JsonNode.
But distinction between "absent" and "explicit 'null' token" required this change.

I am not sure if this is tested and would be interested in seeing if it does work or not yet (there is need for deseralizers to help). Tests for POJOs, enums etc can be found from:

src/test/java/com/fasterxml/jackson/databind/deser/filter/

(like NullConversionsPojoTest.java)

and this also shows how to configure null conversions (via annotations or for ObjectMapper).