Introduce TypeSignature::Comparable and update `NullIf` signature #34970

Job	Run time
build with wasm-pack	2m 49s
cargo check	9m 7s
Check License Header	2m 12s
cargo test (macos)	14m 23s
cargo test (macos-aarch64)	14m 36s
cargo test (win64)	1h 11m 55s
Check cargo fmt	47s
Verify MSRV (Min Supported Rust Version)	8m 46s
Verify Vendored Code	51s
cargo examples (amd64)	14m 5s
cargo test (amd64)	11m 32s
cargo test datafusion-cli (amd64)	4m 11s
cargo test doc (amd64)	16m 42s
check Cargo.toml formatting	2m 56s
clippy	6m 6s
check configs.md and ***_functions.md is up-to-date	3m 25s
cargo test hash collisions (amd64)	17m 51s
cargo doc	4m 24s
Run sqllogictest with Postgres runner	4m 2s
cargo test pyarrow (amd64)	2m 58s
verify benchmark results (amd64)	13m 21s
	3h 46m 59s

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