Rust

Rename `TypeSignature::NullAry` --> `TypeSignature::Nullary` and improve comments #36107

Job	Run time
Check License Header	1m 7s
cargo check	55s
build with wasm-pack	57s
cargo test (macos)	1m 8s
cargo test (win64)	1m 1s
Check cargo fmt	40s
cargo test (macos-aarch64)	1m 7s
Verify MSRV (Min Supported Rust Version)	57s
Verify Vendored Code	49s
cargo test (amd64)	1s
cargo examples (amd64)	1s
cargo test hash collisions (amd64)	1s
clippy	1s
cargo doc	1s
cargo test datafusion-cli (amd64)	1s
cargo test doc (amd64)	1s
Run sqllogictest with Postgres runner	1s
cargo test pyarrow (amd64)	1s
verify benchmark results (amd64)	1s
check Cargo.toml formatting	1s
check configs.md and ***_functions.md is up-to-date	1s
	8m 53s

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