[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.*}

Beginne mit den im Issue genannten Einstiegspunkten BaseWriter, AbstractFieldWriter und UnionListWriter und prüfe anschließend den verknüpften Pull Request zur vorherigen Implementierung. Als abgeschlossen gilt die Arbeit, wenn Nullwerte in komplexen Datentypen die öffentliche BaseWriter.writeNull API verwenden können, ohne auf die standardmäßige IllegalArgumentException zurückzufallen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.