Minor: Use slice in `ConcreteTreeNode` #27130

Job	Run time
build with wasm-pack	1m 59s
cargo check	3m 45s
cargo test (macos)	10m 12s
cargo test (win64)	34m 8s
Check cargo fmt	45s
cargo test (macos-aarch64)	8m 55s
Verify MSRV (Min Supported Rust Version)	6m 33s
Verify Vendored Code	1m 8s
cargo test datafusion-cli (amd64)	3m 53s
cargo examples (amd64)	11m 28s
clippy	5m 4s
cargo doc	3m 34s
cargo test (amd64)	9m 28s
cargo test doc (amd64)	12m 5s
Run sqllogictest with Postgres runner	3m 19s
cargo test pyarrow (amd64)	2m 32s
verify benchmark results (amd64)	10m 55s
check Cargo.toml formatting	3m 4s
check configs.md is up-to-date	2m 50s
cargo test hash collisions (amd64)	8m 51s
	2h 24m 28s

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