release-22.2: sql: fix recently introduced bug around the internal rowsIterator

[yuzefovich]

Backport 2/2 commits from #103514.

/cc @cockroachdb/release

---

This commit fixes a minor bug introduced in c7eb1bf
that could lead to a nil pointer panic. In particular, that commit
introduced HasResults method to the rowsIterator, and it assumed
that first field is always non-nil when the iterator was returned on
QueryIteratorEx call. However, that is not true - in case an error was
returned from the connExecutor goroutine, then rowsIterator.lastErr is
set while first is left nil. The expectation is that in such a case
the user of the iterator will receive that error either on Next or
Close call, properly cleaning up the iterator. We might want to
rethink this and return the error explicitly, but in the spirit of
making the least amount of changes, this commit simply added the non-nil
check for the first field.

Fixes: #103508.

Release note (bug fix): CockroachDB could previously encounter a nil
pointer crash when populating data for SQL Activity page in some rare
cases, and this is now fixed. The bug is present in 22.2.9 and 23.1.1
releases.

Release justification: 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?

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[maryliag]

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