[Java] Introduce BaseWriter.writeNull method

When a data set has null values in complex data type, it's hard to call proper writeNull method. Because writeNull is declared in AbstractFieldWriter, which is package local, not public. Moreover, UnionListWriter has no implementation of writeNull method, so it falls into AbstractFieldWriter.writeNull, which throws an IllegalArgumentException by default. So BaseWriter.writeNull is required to write null values inside of complex data types.

**Reporter**: [Teddy Choi](https://issues.apache.org/jira/browse/ARROW-2538) / @pudidic
#### PRs and other links:
- [GitHub Pull Request apache/arrow#1994](https://github.com/apache/arrow/pull/1994)

_{**Note**: *This issue was originally created as [ARROW-2538](https://issues.apache.org/jira/browse/ARROW-2538). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

まず、issue で指定されているエントリーポイント BaseWriter、AbstractFieldWriter、UnionListWriter から始め、その後、以前の実装についてリンクされた pull request を確認してください。複雑なデータ型の null 値が公開 API である BaseWriter.writeNull を使用でき、デフォルトの IllegalArgumentException にフォールスルーしなくなれば完了です。

索引モデルが issue の本文から書いたものです。