release-25.4: colexec: fix projections with constant NULL values

[blathers-crl]

Backport 1/1 commits from #156962 on behalf of @mgartner.

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This commit fixes a bug in the vectorized engine that caused internal
errors when projections operated on constant NULL values. The
proj_const_right_ops operators do not correctly handle NULL inputs. So,
we avoid these code paths when operator does not operate on NULL values
and the LHS or RHS of an operator is NULL by simply projecting NULLs.
This is similar to the change made in #128123 and addresses a TODO added
in that PR.

This code path is unlikely to hit (maybe impossible) without generic
query plans. In an optimized cusotm plan, the optimizer will fold
expressions with NULL values at optimization time, and the execution
engine will never see expressions of this form.

Fixes #152771

Release note (bug fix): A bug has been fixed that could cause internal
errors for queries using generic query plans with NULL placeholder
values.

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Release justification: Low-risk bug fix.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[cockroach-teamcity]

This change is 

[yuzefovich]

@yuzefovich reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @mgartner and @ZhouXing19)

[michae2]

Ah, I remember #128123, nice!

@michae2 reviewed all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner and @ZhouXing19)