[C++][Parquet] null_count and num_nulls incorrect for repeated columns #42980

Currently only nulls at the leaf are accounted for in the null count statstics.  For nested lists this is incorrect because null lists have zero elements and don't show up in the leaf.

Example from mailing list discussion

[[0, 1], None, [2, None, 3]]

should have a null count of 2 (it currently reports as 1).

Reporter: Micah Kornfield / @emkornfield
Assignee: William Butler / @tachyonwill

PRs and other links:

• GitHub Pull Request #11281

_{Note: This issue was originally created as PARQUET-2067. Please see the migration documentation for further details.}