`Voxels#voxels()` and `voxels_in_range()` no longer return empty voxels

[dylanowen]

I believe the BVH chunking already skips empty sections and when we drop into our VoxelsChunkRef#voxels_in_range() we're dropping empty voxels.

I'm not advocating for reverting this behavior, only returning the non-empty voxels is probably a better API, we'll just want to update the documentation.