Summary:
Allow setting `NULL` into the JSONB column value.
To enable the syntax in the `UPDATE` statement:
```
CREATE TABLE test (id INT PRIMARY KEY, data JSONB);
UPDATE test SET data = null WHERE id = 1;
```

Test Plan: ./yb_build.sh --java-test org.yb.cql.TestJson#testJson

Reviewers: mihnea, stiwary

Reviewed By: stiwary

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D42554