release-22.2.0: colexec: use tree.DNull when projection is called on null input

[blathers-crl]

Backport 1/1 commits from #88425 on behalf of @DrewKimball.

/cc @cockroachdb/release

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Most projections skip rows for which one or more arguments are null, and just output a null for those rows. However, some projections can actually accept null arguments. Previously, we were using the values from the vec even when the Nulls bitmap was set for that row, which invalidates the data in the vec for that row. This could cause a non-null value to be unexpectedly concatenated to an array when an argument was null (nothing should be added to the array in this case).

This commit modifies the projection operators that operate on datum-backed vectors to explicitly set the argument to tree.DNull in the case when the Nulls bitmap is set. This ensures that the projection is not performed with the invalid (and arbitrary) value in the datum vec at that index.

Fixes #87919

Release note (bug fix): Fixed a bug in Concat projection operators for arrays that could cause non-null values to be added to the array when one of the arguments was null.

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Release justification: low-risk fix for rare correctness bug

[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]

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball)

[DrewKimball]

Thanks!

[rytaft]

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball)

[DrewKimball]

TFTR