message: storage: return Ok(None) for check_range_ready_and_mark_pending () when needed
  Return Ok(None) for check_range_ready_and_mark_pending () when the
  result is empty, to follow the definition.
diff: @@ -206,8 +206,12 @@ impl RangeMap for BlobStateMap<IndexedChunkMap, u32> {
  }
  }
  + if res.is_empty() {
  + Ok(None)
  + } else {
  Ok(Some(res))
  }
  + }
  fn set_range_ready_and_clear_pending(&self, start: Self::I, count: Self::I) -> Result<()> {
  let res = self.c.set_range_ready_and_clear_pending(start, count);
  