arrow

GH-34610: [Java] Fix valueCount and field name when loading/transferring NullVector (#38973)

### Rationale for this change

This is a follow up of https://github.com/apache/arrow/pull/34611 (thanks @ sunchao)
We should set `valueCount` when loading `NullVector`. Unfortunately there's no `nullCount` field.

### What changes are included in this PR?

* `valueCount` added to NullVector
* `name` added to NullVector's TransferPair

### Are these changes tested?

Yes. unit tested.

### Are there any user-facing changes?

Yes, a NullVector with a non-null name will maintain the name when transferred via `TransferPair` instead of defaulting to the default name `$data$`.
* Closes: #34610

Authored-by: Chao Sun <sunchao@apple.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>