Added support for explicit null values during deserialization

[timocov]

What behavior changes?

Adds opt-in support for provided null values during the serialization.

Almost no-op if not opted-in (the only change is to call structMemberConsumer.supportsNullValues(member) for each field to check if it was opted-in; by default returns false).

By default, the feature is disabled. But it can be enabled via the codegen integration - a symbol provider should add a SymbolProperties.SUPPORTS_NULL_VALUES property to a member's symbol and set it to true. This will result in generating an override implementation of StructMemberConsumer.supportsNullValues and return true for fields that support it based on the symbol properties. After that, the codegen integration can override a type/serde for such field and, for example, use something like https://docs.vavr.io/#_option instead of a raw type (i.e. Option<String> instead of just String). Then the serde override can check if current value is null, then assign Option.none().

Why is this change needed?

There is a version of application/json protocol that uses "explicit null" values to identify the intent, see https://www.rfc-editor.org/info/rfc7396/.

Currently json-codec skips deserialization of null values. This prevents to detect whether a value was provided as null or wasn't provided at all.

How was this validated?

1. Added tests.
2. Created a simple PoC with overriding a type and the serde codegen for such a field and tested that it gets deserialized correctly (i.e. no value -> null, null as value -> Option.none(), value provided -> Option.some(value)).

What should reviewers focus on?

The logic looks simple, but things worth checking:

• the naming (a new method and a symbol property added)
• whether generateMemberNullValuesSwitchCases should generate cases for every member, not just ones returning true
• it doesn't feel like the perf should be affected, but let me know if you'd want me to run benchmarks
• initially I thought to add another setting to JsonSettings and check it as well, but it felt like the schema/symbols have a better understand if they support null values or not. But let me know if you think that both should be added (so that it would be much cheaper check in the deser if it is not opted-in).

Also, please let me know if this is something you wouldn't want to have a support for.

Additional Links

• https://blog.postman.com/http-patch-method/#section-8:~:text=Handle,field%29%2E
• https://www.rfc-editor.org/info/rfc7396

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.