message: nydusctl: handle prefetch workers is zero as divider
  It is possible prefetch workers is zero when prefetch is disable.
diff: @@ -83,8 +83,11 @@ Persister Buffer: {buffered}
  files = m["underlying_files"],
  directory = m["store_path"],
  requests = m["prefetch_mr_count"],
  - avg_prefetch_size = m["prefetch_data_amount"].as_u64().unwrap()
  - / m["prefetch_mr_count"].as_u64().unwrap(),
  + avg_prefetch_size = m["prefetch_data_amount"]
  + .as_u64()
  + .unwrap()
  + .checked_div(m["prefetch_mr_count"].as_u64().unwrap())
  + .unwrap_or_default(),
  workers = m["prefetch_workers"],
  unmerged_blocks = m["prefetch_unmerged_chunks"],
  buffered = m["buffered_backend_size"],
  