Chunk migration relies on retryable write history recorded via session migration, but this history is currently missing on the recipient for single-operation batched retryable writes because their opTime is only assigned at commit and is therefore not visible on the migration chunk cloner op observer’s per-op onUpdate path.

Multi-op batched writes are handled correctly, but single-op batches are skipped by early returning when  batchOpTimes.empty() because single batched writes record their opTime opAccumulator->opTime rather than in batchOpTimes.

The fix is to relax this guard so we only return early when both batchOpTimes and opTime.writeOpTime are empty, and pass {{opAccumulator->opTime }}for single-op batches so they are correctly added to session migration state.