Fix strpos invocation with dictionary and null #32890

Job	Run time
cargo check	6m 47s
build with wasm-pack	2m 38s
Check cargo fmt	41s
cargo test (macos)	11m 3s
cargo test (macos-aarch64)	12m 20s
Verify MSRV (Min Supported Rust Version)	7m 12s
Verify Vendored Code	49s
cargo test (win64)	57m 8s
cargo examples (amd64)	13m 29s
cargo test (amd64)	9m 56s
cargo test datafusion-cli (amd64)	4m 4s
check Cargo.toml formatting	2m 58s
clippy	5m 41s
check configs.md is up-to-date	2m 53s
cargo test hash collisions (amd64)	15m 4s
cargo test doc (amd64)	15m 7s
Run sqllogictest with Postgres runner	3m 30s
cargo test pyarrow (amd64)	2m 59s
cargo doc	3m 57s
verify benchmark results (amd64)	12m 16s
	3h 10m 32s

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