release-21.2: opt: don't mark st_makeline and st_extend as non-null

[blathers-crl]

Backport 1/1 commits from #86722 on behalf of @DrewKimball.

/cc @cockroachdb/release

---

st_makeline can take arguments of type POINT, MULTIPOINT, and
LINESTRING. Other types cause it to return null (even if the input is
non-null). Similarly, st_extent returns null when the input geometry is
non-null, but empty.

This commit prevents these two aggregate functions from being marked as
non-null, since doing so can lead to incorrect results in the cases
mentioned above.

Fixes #84957

Release justification: low-risk fix to bug that can lead to incorrect results

Release note (bug fix): Fixed a longstanding bug that could cause the optimizer
to produce an incorrect plan when aggregate functions st_makeline or
st_extent were called with invalid-type and empty inputs respectively.

---

[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 

[rytaft]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)