Fix count on null values #275

Job	Run time
cargo test (macos)	5m 2s
cargo check	6m 40s
build with wasm-pack	2m 41s
Check cargo fmt	48s
cargo test (macos-aarch64)	5m 12s
Verify Vendored Code	59s
cargo test (win64)	11m 11s
Verify MSRV (Min Supported Rust Version)	8m 8s
cargo test (amd64)	7m 4s
cargo examples (amd64)	13m 22s
cargo test datafusion-cli (amd64)	3m 50s
cargo doc	3m 52s
cargo test doc (amd64)	15m 30s
Run sqllogictest with Postgres runner	3m 29s
verify benchmark results (amd64)	12m 34s
check Cargo.toml formatting	2m 45s
clippy	5m 45s
check configs.md and ***_functions.md is up-to-date	3m 3s
cargo test hash collisions (amd64)	6m 48s
cargo test pyarrow (amd64)	2m 55s
	2h 1m 38s

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