[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.*}

从 Java Avro-to-Arrow 转换器开始，使用 ARROW-11629 中引用的 CSV 文件重现该问题。检查链接 gist 中的残留内容，重点关注 union 处理，然后将生成的数据与 C++ 实现所接受的数据进行比较。当转换器生成可由 C++ 实现读取的 Arrow 数据，并为失败的输入添加回归测试时，即视为完成。

由索引模型根据 Issue 内容生成。