[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.*}

Start at NullableMapVector.getField and compare it with the superclass implementation. Confirm that the method delegates to super.getField() and returns a nullable version of that field; add or update focused coverage if the repository provides a relevant test.

Written by the indexing model from the issue text.