sql: fix recently introduced bug around the internal rowsIterator

[yuzefovich]

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.

[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]

Oh thanks for fixing this!!

Reviewed 1 of 1 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

[yuzefovich]

TFTR!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from c0a20cd to blathers/backport-release-22.2-103514: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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