Support `IS NULL` and `IS NOT NULL` on Unions #28981

Job	Run time
build with wasm-pack	2m 24s
cargo test (win64)	33m 33s
cargo check	7m 49s
cargo test (macos)	9m 58s
Check cargo fmt	40s
cargo test (macos-aarch64)	8m 53s
Verify Vendored Code	54s
Verify MSRV (Min Supported Rust Version)	6m 27s
cargo examples (amd64)	12m 44s
cargo test (amd64)	9m 4s
clippy	5m 0s
cargo doc	3m 31s
cargo test hash collisions (amd64)	8m 59s
cargo test datafusion-cli (amd64)	3m 46s
cargo test doc (amd64)	12m 45s
Run sqllogictest with Postgres runner	3m 9s
cargo test pyarrow (amd64)	2m 34s
verify benchmark results (amd64)	10m 18s
check Cargo.toml formatting	2m 56s
check configs.md is up-to-date	2m 29s
	2h 27m 53s

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