Minor optimization of the null handling in ArrayAggAccumulator #1302

Job	Run time
cargo check	8m 24s
build with wasm-pack	2m 20s
cargo test (macos)	11m 17s
Check cargo fmt	27s
cargo test (macos-aarch64)	10m 30s
Verify Vendored Code	52s
cargo test (win64)	33m 29s
Verify MSRV (Min Supported Rust Version)	4m 0s
cargo examples (amd64)	11m 51s
cargo test doc (amd64)	12m 27s
cargo test datafusion-cli (amd64)	3m 36s
cargo test (amd64)	9m 11s
cargo doc	3m 54s
verify benchmark results (amd64)	10m 38s
check Cargo.toml formatting	2m 58s
clippy	1m 57s
check configs.md is up-to-date	2m 32s
cargo test hash collisions (amd64)	2m 19s
Run sqllogictest with Postgres runner	3m 12s
cargo test pyarrow (amd64)	2m 49s
	2h 18m 43s

Provide feedback