GH-36053: [C++] summarizing a variable results in NA at random, while there is no NA in the subset of data

[westonpace]

Rationale for this change

When merging two aggregate states we were failing to use the correct no_nulls field. This field tells us whether we should return null if skip_nulls=False (if no_nulls is false then we return null).

Since we were reading the wrong field we would sometimes emit null even when a column didn't actually have any nulls.

What changes are included in this PR?

Fixed the bug.

Are these changes tested?

Yes, I added a new unit test that reproduced this failure quite reliably.

Are there any user-facing changes?

No.

• Closes: [C++][R] summarizing a variable results in NA at random, while there is no NA in the subset of data. #36053

[github-actions]

⚠️ GitHub issue #36053 has been automatically assigned in GitHub to PR creator.

[bkietz]

LGTM

[paleolimbot]

Just a note that I ran the original R reprex against Arrow C++ built from this branch to confirm that it fixes the issue (it does! Thank you!).

[conbench-apache-arrow]

Conbench analyzed the 6 benchmark runs on commit 0cea12ff.

There were 6 benchmark results indicating a performance regression:

• Commit Run on arm64-m6g-linux-compute at 2023-06-29 19:56:15Z

  • params=<Round, FloatType, RoundMode::HALF_TOWARDS_ZERO>/size:1048576/inverse_null_proportion:100, source=cpp-micro, suite=arrow-compute-scalar-round-benchmark

• Commit Run on ursa-thinkcentre-m75q at 2023-07-03 10:15:31Z

  • params=1024, source=cpp-micro, suite=parquet-encoding-benchmark

• and 4 more (see the report linked below)

The full Conbench report has more details.