[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.*}

Comece pelos pontos de entrada BaseWriter, AbstractFieldWriter e UnionListWriter mencionados na issue e, em seguida, revise o pull request vinculado referente à implementação anterior. O trabalho estará concluído quando valores null em tipos de dados complexos puderem usar a API pública BaseWriter.writeNull sem cair na IllegalArgumentException padrão.

Escrita pelo modelo de indexação a partir do texto da issue.