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James Henderson authored and GitHub committed 515c61dd617
GH-40773: [Java] add `DENSEUNION` case to StructWriters, resolves #40773 (#40809) ### What changes are included in this PR? Adding a `DENSEUNION` case to the `StructWriters` template so that one can create StructVectors with a DenseUnionVector child. ### Are these changes tested? Yes ### Are there any user-facing changes? No * GitHub Issue: #40773 Authored-by: James Henderson <james@jarohen.dev> Signed-off-by: David Li <li.davidm96@gmail.com>