message: skip computation if matrix is zero
diff: @@ -618,6 +618,8 @@ void SparseMatrix::AddMult(const Vector &x, Vector &y, const double a) const
  auto d_x = x.Read();
  auto d_y = y.ReadWrite();
  + //Skip if matrix is all zero
  + if (nnz == 0) {return;}
  #ifdef MFEM_USE_CUDA
  if (Device::Allows(Backend::CUDA_MASK))
  {
  