release-25.4: sql/stats: handle range counts when skipping dropped enum hist buckets

[michae2]

Backport 1/2 commits from #155035.

/cc @cockroachdb/release

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opt/props: only check histogram NumRange=0 in test builds

These assertions that the first histogram bucket has NumRange=0 are
useful for catching bugs, but a liability in production
environments. Even with malformed histograms, we should not be failing
query execution. Only check these assertions in test builds.

Informs: #154461

Release note (bug fix): Change assertions about histogram NumRange=0 to
only be checked in test builds.

---

Release justification: low-risk fix for a critical support issue.

[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.

[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 

[yuzefovich]

@yuzefovich reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @michae2)