feat: Don't allocate another DType in `DType::eq_ignore_nullability` #6632

Job	Run time
Python (lint)	1m 13s
Rust (docs)	47s
Rust build (all-features)	45s
bench test	1m 15s
Rust build (default features)	1m 12s
License Check and Audit Check	1m 14s
Rust build (with tokio dispatcher)	1m 13s
Check generated proto/fbs files are up to date	1m 2s
miri	1m 12s
Rust build (with compio dispatcher)	1m 13s
Rust (lint)	1m 2s
Python (test)	1m 12s
Rust build (with tokio+compio dispatcher)	1m 11s
Rust (tests)	1m 12s
Rust build (wasm32 with default features)	1m 12s
	16m 55s

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