Fix segfault in poisson disk downsampling of empty point cloud

[SAY-5]

Fixes #115.

downsample_point_cloud_poisson_disk crashes when the input point cloud has zero points. The radius path calls blue_noise_downsample, which computes grid bounds with X.colwise().minCoeff() and Xs.maxCoeff(); both are undefined on a zero-row Eigen matrix, so the empty case reads out of bounds and segfaults.

This returns an empty index array immediately when the input has no points, before the bounds are computed. Added a regression test for the empty-input case.

[fwilliams]

Thanks so much for the fix! I'll approve once CI goes green

[SAY-5]

The ubuntu-22.04 wheel job failed while building the scipy build-dependency from source (metadata generation for scipy under cibuildwheel), which is unrelated to the two files changed here. A re-run should pick it up once the manylinux image resolves a buildable scipy; happy to rebase onto main if you'd prefer to retrigger that way.

[fwilliams]

Yes, could you rebase onto main? Thanks a lot!

[SAY-5]

Rebased onto the latest master, so CI should pick up a fresh manylinux image now. Thanks!