CIBW_ARCHS should be allowed to be empty

[henryiii]

Description

In cibuildwheel, we treat empty environments variables and unset environment variables identically in most cases. We don't seem to do this for CIBW_ARCHS, though, as setting CIBW_ARCHS: ${{ matrix.archs }} doesn't work if archs is not set. I think it should wok like the others.

Build log

https://github.com/pybind/scikit_build_example/actions/runs/16600944084/job/46960140697

CI config

https://github.com/pybind/scikit_build_example/blob/f9b0f810f3fd229383bb3b5025547ecb80818e53/.github/workflows/cibw-cc.yaml