message: fix Csr initialization of empty load_balance mtx
diff: @@ -433,10 +433,12 @@ public:
  }
  const auto num_rows = mtx_row_ptrs.get_num_elems() - 1;
  const auto num_elems = row_ptrs[num_rows];
  + const auto bucket_divider =
  + num_elems > 0 ? ceildiv(num_elems, warp_size_) : 1;
  for (size_type i = 0; i < num_rows; i++) {
  auto bucket =
  ceildiv((ceildiv(row_ptrs[i + 1], warp_size_) * nwarps),
  - ceildiv(num_elems, warp_size_));
  + bucket_divider);
  if (bucket < nwarps) {
  srow[bucket]++;
  }
  