message: builder: fix panic caused by empty blob entry in blob table
  Empty blob table has zero size data, so size() method on blob table
  should return 0 directly, otherwise will cause panic on `.fold(...) - 1`.
diff: @@ -556,6 +556,9 @@ impl OndiskBlobTable {
  /// Get blob table size, aligned with RAFS_ALIGNMENT bytes
  pub fn size(&self) -> usize {
  + if self.entries.is_empty() {
  + return 0;
  + }
  // Blob entry split with '\0'
  align_to_rafs(
  self.entries
  