release-26.1: sql/stats: nil-check cache in TableStatisticsCache.Stop

[blathers-crl]

Backport 1/1 commits from #169624 on behalf of @SAY-5.

---

Fixes #169306

The closer registered in NewTableStatisticsCache calls tableStatsCache.Stop. The closer is registered before sc.mu.cache is initialised on line 310. If the parent stopper begins draining closers during cache construction (rare, but observed in TestServerController on shared-process tenants), Stop fires on a partially-initialised cache and sc.mu.cache.Clear() nil-derefs. Guard the Clear call with a nil check; acc.Clear and mon.Stop are already either no-ops or guarded.

Release note: None

---

Release justification: fix a missing nil check that could cause panic on user end.

[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.

[trunk-io]

Merging to release-26.1 in this repository is managed by Trunk.

• To merge this pull request, check the box to the left or comment /trunk merge below.

After your PR is submitted to the merge queue, this comment will be automatically updated with its status. If the PR fails, failure details will also be posted here

[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 

[blathers-crl]

Detected infrastructure failure (matched: self-hosted runner lost communication with the server). Automatically rerunning failed jobs. (run link)