[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.*}

Comienza con los puntos de entrada BaseWriter, AbstractFieldWriter y UnionListWriter mencionados en el issue, y después revisa el pull request enlazado para consultar la implementación anterior. Se considera terminado cuando los valores null en tipos de datos complejos pueden usar la API pública BaseWriter.writeNull sin terminar en la IllegalArgumentException predeterminada.

Escrito por el modelo de indexación a partir del texto del issue.