feat: Allow null matrix entries.

[bdice]

This PR allows null matrix entries to co-exist with non-null matrix entries.

Currently a matrix like

files:
  all:
    output: conda
    matrix:
      cuda: ["11.8", "12.5"]
      arch: [x86_64]

implies that undefined variables like py: are ignored.

With this PR, you can add a line like

files:
  all:
    output: conda
    matrix:
      cuda: ["11.8", "12.5"]
      arch: [x86_64]
      py: [null, "3.13t"]

to generate the same environments as before (where py was undefined) and additionally generate environments where py: "3.13t" is set.

I retained the naming behavior so that if a key is null, it does not appear in the environment name (we get all_cuda-125_arch-x86_64.yaml instead of all_cuda-125_arch-x86_64_py-None.yaml because I thought the py-None was an unwanted change).

An alternative to this would be to specify a different file key where the new matrix key is defined, but that would require replicating the includes: list.

[vyasr]

@bdice is this ready for a review?

[bdice]

@vyasr Yes, it is ready for review.

[vyasr]

Seems fine to me. Agree with the choice of retaining the current naming scheme.

[GPUtester]

🎉 This PR is included in version 1.17.0 🎉

The release is available on:

• GitHub release
• PyPI release
• Conda release - rapidsai
• Conda release - rapidsai-nightly

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