release-22.2: colbuilder: fix aggregation with no aggregate funcs

[blathers-crl]

Backport 1/1 commits from #87662 on behalf of @yuzefovich.

/cc @cockroachdb/release

---

Previously, if there are no aggregate functions to compute, we would plan a special "num fixed tuples" operator that would always return a single tuple when in scalar and no tuples when in non-scalar context, but this is incorrect - we should be returning the same number of tuples as there are groups for non-empty input. The previous setup was correct only when the input is empty. This is now fixed by teaching the ordered aggregator to handle this special case instead. The impact of the bug seems minor since the optimizer should only be creating such plans when some of its rules are disabled.

Fixes: #87619.

Release note: None (this shouldn't be a user-visible bug)

---

Release justification: low-risk bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[yuzefovich]

This can wait for 22.2.1.

[msirek]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball and @yuzefovich)

[yuzefovich]

@mgartner WDYT about backporting this to 22.2 right now rather than waiting for 22.1.1? It seems like it would squash some of the flakes on that branch and it's a low-risk change. At the same time we are not aware of any real bugs this would fix nor is it a regression.

[mgartner]

@yuzefovich I was thinking the same thing. Let's do it. I'll approve in #release-backports if you submit it.