[Java] Fix getField() for NullableMapVector

Needs to call super.getField() and return a nullable version of that field.

**Reporter**: [Steven Phillips](https://issues.apache.org/jira/browse/ARROW-790) / @StevenMPhillips

_{**Note**: *This issue was originally created as [ARROW-790](https://issues.apache.org/jira/browse/ARROW-790). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

从 NullableMapVector.getField 开始，并将其与超类实现进行比较。确认该方法委托给 super.getField()，并返回该字段的 nullable 版本；如果 repository 提供了相关测试，请添加或更新有针对性的覆盖范围。

由索引模型根据 Issue 内容生成。