Add subtrait support for `IS NULL` and `IS NOT NULL` #19984

Job	Run time
Check cargo fmt	39s
cargo check	3m 51s
cargo test (mac)	19m 47s
build with wasm-pack	3m 2s
Verify MSRV	10m 18s
Verify Vendored Code	48s
cargo test (win64)	30m 31s
cargo test (amd64)	13m 39s
cargo test datafusion-cli (amd64)	5m 14s
cargo doc	5m 56s
cargo test doc (amd64)	9m 20s
cargo test hash collisions (amd64)	8m 34s
cargo examples (amd64)	9m 42s
clippy	9m 3s
cargo test pyarrow (amd64)	2m 9s
Run sqllogictest with Postgres runner	5m 13s
verify benchmark results (amd64)	9m 25s
check configs.md is up-to-date	3m 20s
check Cargo.toml formatting	1m 27s
	2h 31m 58s

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