[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.*}

Inizia dai punti di ingresso BaseWriter, AbstractFieldWriter e UnionListWriter indicati nell’issue, quindi esamina la pull request collegata per la precedente implementazione. Il lavoro è completato quando i valori null nei tipi di dati complessi possono usare l’API pubblica BaseWriter.writeNull senza ricadere nell’IllegalArgumentException predefinita.

Scritto dal modello di indicizzazione a partire dal testo della issue.