POC: Remove unnecessary null check for `GroupColumn` #33551

Job	Run time
cargo check	1m 5s
build with wasm-pack	2m 2s
cargo test (macos)	2m 0s
cargo test (macos-aarch64)	2m 4s
cargo test (win64)	6m 18s
Check cargo fmt	38s
Verify MSRV (Min Supported Rust Version)	5m 37s
Verify Vendored Code	55s
cargo test datafusion-cli (amd64)	0s
cargo test (amd64)	0s
cargo test doc (amd64)	0s
cargo doc	0s
cargo examples (amd64)	0s
check Cargo.toml formatting	0s
clippy	0s
check configs.md and ***_functions.md is up-to-date	0s
cargo test hash collisions (amd64)	0s
Run sqllogictest with Postgres runner	0s
cargo test pyarrow (amd64)	0s
verify benchmark results (amd64)	0s
	20m 39s

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