Empty reference distances in checkResults.

Created by: masterleinad

As discovered in https://github.com/arborx/ArborX/pull/70#issuecomment-497903087,

template <typename Query, typename DeviceType>
void checkResults(ArborX::DistributedSearchTree<DeviceType> const &tree,
                  Kokkos::View<Query *, DeviceType> const &queries,
                  std::vector<int> const &indices_ref,
                  std::vector<int> const &offset_ref,
                  std::vector<int> const &ranks_ref,
                  std::vector<double> const &distances_ref)

is always run with empty distances_ref.