Change `ObjectMapper.valueToTree()` to convert `null` to `NullNode`

[cowtowncoder]

It looks like valueToTree() method will currently simple return null if null is passed.
For many other conversions this makes sense, but in case of JsonNode, it would make more sense to return NullNode.

[asnare]

Interestingly, I think this change leads to a subtle behaviour change when combined with Jdk8Module. We have a test for a @JsonCreator method that has an Optional<JsonNode> parameter. Prior to this change a field that wasn't present would get mapped to Optional.empty(), whereas now it gets mapped to Optional.of(NullNode).

I guess it's splitting hairs, but previously this meant it was possible to differentiate between an attribute that was missing entirely and one that was present but with null assigned. Now they seem to be treated as exactly equivalent. (Is this correct?)

[cowtowncoder]

@asnare Hmmh. So, two things should not be directly related: treeToValue() has specific handling where null from JsonParser is received. It should not be used by code that handles @JsonCreator values: that should just get null. I do agree that Optional<JsonNode from null token should become Optional.empty(), and not Optional.of(nullNode). Question then is... why did behavior change?

IF you can easily reproduce this, could you please file an issue against jackson-modules-java8?