feat: teach SparseArray maybe_null_indices_iter #4502

Job	Run time
Build (default)	4m 14s
bench test	16m 10s
Build (all-features)	2m 59s
Check generated proto/fbs files are up to date	1m 2s
miri	9m 34s
License Check and Audit Check	1m 40s
Python (lint)	3m 23s
Python (test)	6m 7s
Rust (docs)	1m 0s
Rust (tests)	6m 10s
	52m 19s

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