bulk: audit empty-instance handling for execution locality across bulk jobs

[himanshu-shrivastava-crl]

Summary:
Bulk jobs that support an execution-locality filter plan their worker set via
dsp.SetupAllNodesPlanningWithOracle(... SingleLocalityFilter ...), which returns
an empty instance slice (nil error) when no live instance matches the filter.
Callers must guard this empty case; not all do.

Concrete finding:
restore_processor_planning.go computes
chunkSize := int(math.Sqrt(float64(md.numImportSpans))) / numNodes with no guard
against numNodes == 0. A RESTORE execution-locality filter matching zero live
instances divides by zero (panic). IMPORT guards this explicitly ("no SQL
instances match ..."); other callers should be checked.

Next steps:

• Audit each SetupAllNodesPlanningWithOracle caller (backup, restore,
  compaction, revlog, merge, fingerprint) for empty-instance handling.
• Fix the RESTORE divide-by-zero.
• Consider centralizing the empty guard in the planning helper.

Epic CRDB-65504

Jira issue: CRDB-66930

[blathers-crl]

cc @cockroachdb/disaster-recovery