Use `@JsonSetter(nulls=...)` to specify handling of `null` values during deserialization

[cowtowncoder]

(note: replaces #1269, earlier #988 and #830)

It looks like there is strong desire to define alternate methods for handling incoming null values; instead of just assigning as Java nulls to:

1. Ignore value (avoid calling setter)
2. Fail (throw exception)
3. Convert to a default value

Since applicability is probably not universal for all cases (that is, different types could well follow different rules), it seems that approach similar to @JsonInclude would work.
However, I don't think use of @JsonInclude itself works here, as its settings are partially output-specific; so ideas are reusable, implementation not.

Instead, I think use of existing @JsonSetter by adding:

• Nulls value enumeration for a small set of standard behaviors (and perhaps, in future, for custom handler?)
• nulls property for dealing with value itself (POJO, Collection, wrapper type)
• contentNulls for contents of structured types: array/collection elements, Map values, POJO properties

Changes to annotation are simple; changes to deserializers would be sizable.

[raindev]

Is there any ongoing progress to be able to deserialize nulls to empty collections? We regularly suffer from NullPointerExecptions in our API clients. It would be great to solve the issue systematically.

[cowtowncoder]

@raindev This would be option (3), although it may be that most users would prefer initializing empty collection values in constructor, and opt for simple skip (1) instead. Either way could solve the null problem; former would require bit more complex handling (deserializer implementation would need to know how to produce default value; and it may or may not be easily accessible).

[raindev]

@cowtowncoder, but as I understand it is not implemented yet, right? Can I help, if it is not?

[cowtowncoder]

@raindev It's very high on my todo list, but correct, it is not yet implemented.

Help is always welcome, although in this case I do have reasonable idea of how to proceed: basically it would use approach somewhat similar to #1399, modifying SettableBeanProperty to skip setting. There are some open questions on interaction for obtaining "empty" instance (if that is to be supported).

For what it is worth, annotation side has already been added, so information is already available during construction of BeanDeserializer / SettableBeanProperty.

So... if you want to have a look, feel free to: I rarely keep much uncommitted code locally, so everything implemented by me so far is in master. Rest are just ideas which I am happy to share.