fix: handle zero-length null extension for empty unions - #11209

closing as i suspect its AI spam and doesnt fully address the root cause issue

Per discussion on #11215, reopening this PR and marking it draft while @efegokdemir works through the feedback

Addressed the non-zero case in fa9d2110. Empty unions now return InvalidArgumentError before mutating state when a positive null extension is requested; zero-length extensions remain no-ops for both sparse and dense unions. The regression test covers both modes and verifies the length remains unchanged after the error.

Validation: /opt/homebrew/bin/rustfmt --check arrow-data/src/transform/mod.rs arrow-data/src/transform/union.rs, git diff --check, and cargo test --locked --manifest-path arrow-data/Cargo.toml -p arrow-data --lib test_try_extend_nulls_empty_union all passed.