[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.*}

先从 issue 中提到的 BaseWriter、AbstractFieldWriter 和 UnionListWriter 入口点开始，然后查看链接的 pull request 以了解之前的实现。当复杂数据类型中的 null 值可以使用公开的 BaseWriter.writeNull API，而不会落入默认的 IllegalArgumentException 时，即可视为完成。

由索引模型根据 Issue 内容生成。