message: avoid copies on empty matrices
  This fixes test failures in core tests with Reference disabled
diff: @@ -331,7 +331,7 @@ void Dense<ValueType>::compute_norm2_impl(LinOp *result) const
  template <typename ValueType>
  void Dense<ValueType>::convert_to(Dense<ValueType> *result) const
  {
  - if (result->get_size() == this->get_size()) {
  + if (this->get_size() && result->get_size() == this->get_size()) {
  // we need to create a executor-local clone of the target data, that
  // will be copied back later.
  auto exec = this->get_executor();
  