SPC: support creation with empty octrees.

[orperel]

Creating a SPC with an empty octree of 0 LODs (octree is a tensor of [0]) fails.

Suggested fix: update kaolin.ops.spc.scan_octrees to return an empty torch.Tensor when octrees is a tensor of [0].

[sparse-mvs-2]

I face the same problem. Besides I found that spc at any level of LOD with octrees all empty ( that is, spc.octrees = [0,0,0,0,...,0]) is not supported. Now I have to judge this situation by summation.