message: Fix nullptr returned by prefix interface
diff: @@ -24,7 +24,7 @@ void SnapshotManagerImpl::accessAllRowsInSnapshot(GraphSpaceID spaceId,
  int32_t batchSize = 0;
  int64_t totalSize = 0;
  int64_t totalCount = 0;
  - while (iter->valid()) {
  + while (iter && iter->valid()) {
  if (batchSize >= FLAGS_snapshot_batch_size) {
  cb(std::move(data), totalCount, totalSize, false);
  data.clear();
  