Changes

Exit early from unionFindWithingEachDistanceCell to avoid bounds check 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.