arrow

MINOR: [Java] `DenseUnionVector.empty` should create not-nullable DUVs (#41001)

### Rationale for this change

DUVs do not have a validity vector, so cannot be set to null - `isNull`, for example, always returns false. This change ensures vectors created through `DenseUnionVector.empty` reflect this in their FieldType.

### What changes are included in this PR?

`DenseUnionVector.empty` now creates DUVs with a not-nullable `FieldType`

### Are these changes tested?

I haven't added an explicit test for this as it would essentially be testing `FieldType.notNullable` - confirmed that it doesn't break any existing tests.

### Are there any user-facing changes?

Yes, strictly speaking this is a public change, correcting a bug in a public API.

**This PR includes breaking changes to public APIs.**

Authored-by: James Henderson <james@jarohen.dev>
Signed-off-by: David Li <li.davidm96@gmail.com>