[Java] Minor types don't account for nullable FieldType flag

Calling e.g. `FLOAT4.getNewVector("foo", new FieldType(false, ...), ...)" returns a NullableFloat4Vector instead of a Float4Vector.
edit: Float4Vector doesn't implement FieldVector, so can't currently be a top-level vector. I'm confused as to what the nullable flag is supposed to represent then.

**Reporter**: [Emilio Lahr-Vivaz](https://issues.apache.org/jira/browse/ARROW-999) / @elahrvivaz

_{**Note**: *This issue was originally created as [ARROW-999](https://issues.apache.org/jira/browse/ARROW-999). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Comece pela implementação Java de FLOAT4.getNewVector e inspecione como FieldType(false, ...) é interpretado para Float4Vector e NullableFloat4Vector. Rastreie a restrição de FieldVector mencionada na issue e determine o significado pretendido do sinalizador nullable; considera-se concluído quando o tipo do vetor retornado e o comportamento do vetor de nível superior correspondem a esse contrato.

Escrita pelo modelo de indexação a partir do texto da issue.