Handle jobs with `"result": null,`

[webknjaz]

This can happen if GHA definition is borked partially and GHA kinda skips a dependent job but doesn't mark it as such really.

    "build_windows": {
      "result": null,
      "outputs": {}
    },

(https://github.com/python/cpython/actions/runs/9950331379/job/27488343862?pr=121831#step:2:62)

I think, we should output some hint informing the end-users that they need to review their workflow.