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GH-44353: [Java] Implement `map()` for `UnionMapWriter` (#44390)

### Rationale for this change

See #44353 

`UnionMapWriter` does not override `map()` and falls back to implementation in `UnionListWriter` which causes exception when used.

### What changes are included in this PR?

Implement `map()` for `UnionMapWriter`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Changed behavior of the related function. 

* GitHub Issue: #44353

Authored-by: 0xderek <allenjuly7@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>