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GH-40796: [Java] set `lastSet` in `ListVector.setNull` to avoid O(n²) in ListVectors with lots of nulls (#40810)

Would benefit from someone with knowledge of the context double-checking this doesn't have nuances I'm not aware of - particularly, there's a comment on the field: `the maximum index that is actually set` which one _could_ read to mean 'excluding nulls'?

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: #40796

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