Add empty string to string test data #310

Job	Run time
cargo check	11m 21s
build with wasm-pack	2m 41s
cargo test (macos)	8m 3s
cargo test (macos-aarch64)	7m 16s
Verify Vendored Code	51s
cargo test (win64)	57m 37s
Check cargo fmt	45s
Verify MSRV (Min Supported Rust Version)	8m 52s
cargo test (amd64)	10m 44s
cargo doc	4m 19s
cargo test datafusion-cli (amd64)	3m 53s
cargo test doc (amd64)	15m 43s
cargo examples (amd64)	13m 49s
Run sqllogictest with Postgres runner	3m 41s
cargo test pyarrow (amd64)	2m 57s
verify benchmark results (amd64)	12m 24s
check Cargo.toml formatting	2m 59s
clippy	5m 47s
check configs.md and ***_functions.md is up-to-date	3m 11s
cargo test hash collisions (amd64)	16m 25s
	3h 13m 18s

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