Exit early from unionFindWithingEachDistanceCell to avoid bounds chec…

…k trigger

When Kokkos is compiled with Kokkos_ENABLE_DEBUG_BOUNDS_CHECK=ON,
examples/dbscan reports
```
1: Kokkos::RangePolicy bounds error: The lower bound (1) is greater than the upper bound (0).
```
This is coming from this function, where RangePolicy starts at 1. This
is a minor change to exit early. An additional benefit is not launching
the kernel when n == 0.

src/details/ArborX_DetailsFDBSCANDenseBox.hpp

@@ -274,14 +274,17 @@ void unionFindWithinEachDenseCell(ExecutionSpace const &exec_space,
                                   CellIndices sorted_dense_cell_indices,
                                   Permutation permute, UnionFind union_find)
 {
+  auto const n = sorted_dense_cell_indices.size();
+  if (n <= 1)
+    return;
+
   // The algorithm relies on the fact that the cell indices array only contains
   // dense cells. Thus, as long as two cell indices are the same, a) they
   // belong to the same cell, and b) that cell is dense, thus they should be in
   // the same cluster. If, on the other hand, the array also contained
   // non-dense cells, that would not have been possible, as an additional
   // computations would have to be done to figure out if the points belong to a
   // dense cell, which would have required a linear scan.
-  auto const n = sorted_dense_cell_indices.size();
   Kokkos::parallel_for(
       "ArborX::DBSCAN::union_find_within_each_dense_box",
       Kokkos::RangePolicy<ExecutionSpace>(exec_space, 1, n),