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

[DrewKimball]

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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.

[cockroach-teamcity]

This change is 

[DrewKimball]

TFTR! bors r+

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)

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error creating merge commit from 770898abe8471b62453b43789fd983ecb1af2494 to blathers/backport-release-21.2-88425: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

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error creating merge commit from 770898abe8471b62453b43789fd983ecb1af2494 to blathers/backport-release-22.1-88425: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

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Backport to branch 22.1.x failed. See errors above.

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[DrewKimball]

blathers backport 22.2.0