[ntuple] RValue::GetRef<T> should type-check

[hahnjo]

Using RNTupleView<void>, it is possible to write the following code:

#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>
#include <ROOT/RNTupleView.hxx>

#include <iostream>

void ntuple_view_void() {
  {
    auto model = ROOT::RNTupleModel::Create();
    *model->MakeField<float>("px") = 42.0f;
    auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", "void.root");
    writer->Fill();
  }

  auto reader = ROOT::RNTupleReader::Open("ntpl", "void.root");
  auto view = reader->GetView<void>("px");
  view(0);
  auto value = view.GetValue().GetRef<double>();
  std::cout << "value = " << value << "\n";
}

This should throw when trying to get a double reference to a float object.