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GH-37014: [C++][Parquet] Preserve some Parquet distinct counts when merging stats (#37016)

### Rationale for this change

Statistics merging is unnecessarily conservative with respect to preserving distinct counts. If either side is 0, the merge stats can preserve the distinct count, instead of clearing it.

### What changes are included in this PR?

Distinct count is preserved if either the lhs or rhs has a distinct count of 0 when merging stats.

### Are these changes tested?

Yes, added a unit test.

### Are there any user-facing changes?

I don't think this change to stats is considered to be user-facing.
* Closes: #37014

Lead-authored-by: Elliott Brossard <elliott.brossard@snowflake.com>
Co-authored-by: Elliott Brossard <64754120+sfc-gh-ebrossard@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>