Fix a minor EXPLAIN bug around count(*)

[ggevay]

See https://github.com/MaterializeInc/database-issues/issues/8831#issuecomment-2643526472

The first commit is just a 1-liner fix for the bug (and adding tests), while the second commit is a bit of refactoring to simplify some related code.

Motivation

• This PR fixes a recognized bug: https://github.com/MaterializeInc/database-issues/issues/8831#issuecomment-2643526472

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[ParkMyCar]

Sweet, thanks Gabor!