release-25.4: sql/stats: nil-check cache in TableStatisticsCache.Stop

[blathers-crl]

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

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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

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

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[blathers-crl]

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[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?

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[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)

[blathers-crl]

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