[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.*}

Commencez par les points d’entrée BaseWriter, AbstractFieldWriter et UnionListWriter mentionnés dans l’issue, puis examinez la pull request liée concernant l’implémentation précédente. Le travail est terminé lorsque les valeurs null dans les types de données complexes peuvent utiliser l’API publique BaseWriter.writeNull sans retomber sur l’IllegalArgumentException par défaut.

Rédigé par le modèle d'indexation à partir du texte de l'issue.