Fix rendering of dictionary empty string values in SLT tests #309

Job	Run time
build with wasm-pack	2m 41s
Check cargo fmt	42s
cargo test (macos)	10m 40s
cargo check	11m 26s
cargo test (macos-aarch64)	12m 15s
Verify MSRV (Min Supported Rust Version)	9m 11s
Verify Vendored Code	50s
cargo test (win64)	1h 3m 58s
cargo examples (amd64)	13m 44s
cargo test datafusion-cli (amd64)	4m 2s
cargo test (amd64)	11m 12s
cargo doc	4m 4s
cargo test doc (amd64)	15m 48s
Run sqllogictest with Postgres runner	3m 35s
verify benchmark results (amd64)	12m 55s
cargo test pyarrow (amd64)	2m 52s
check Cargo.toml formatting	3m 4s
clippy	5m 51s
check configs.md and ***_functions.md is up-to-date	3m 10s
cargo test hash collisions (amd64)	16m 4s
	3h 28m 4s

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