[Java] Avro to Arrow converter doesn't appear to generate valid arrow data

I think this is related to how Unions are handled (I had thought unions of with a null and one other type would get created to the nullable type, but that is a separate issue).

I haven't had time to fully diagnose, but remnants of the code I tried to use are at

And the CSV file from https://issues.apache.org/jira/browse/ARROW-11629?jql=text%20~%20%22arrow%20drill%20parquet%20dictionary%22

produce data that isn't readable by the C++ implementation.

**Reporter**: [Micah Kornfield](https://issues.apache.org/jira/browse/ARROW-12340) / @emkornfield

_{**Note**: *This issue was originally created as [ARROW-12340](https://issues.apache.org/jira/browse/ARROW-12340). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the Java Avro-to-Arrow converter and reproduce the problem using the CSV file referenced from ARROW-11629. Review the remnants in the linked gist, focusing on union handling, then compare the generated data with what the C++ implementation accepts. Done means the converter produces Arrow data readable by the C++ implementation, with a regression test for the failing input.

Written by the indexing model from the issue text.