[FLINK-26374][table-planner] Support nullability in nested JSON_OBJECT values

[twalthr]

What is the purpose of the change

Nullability was treated properly when serializing nested values into JSON.

Brief change log

• Add null checking logic that translates to Jackson's nullNode if necessary.

Verifying this change

This change added tests and can be verified as follows: JsonFunctionITCase

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• d381c14 Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[Airblader]

LGTM. Thanks for fixing this. 🙃