Use "omit" instead of "null" to skip compose

[spetrosi]

Using null doesn't work because

Using null as suggested by current readme fails with the following error. An example run is here: https://github.com/redhat-cop/infra.leapp/actions/runs/19141825777

Run sclorg/testing-farm-as-github-action@v4
Error: [
  {
    "code": "invalid_type",
    "expected": "object",
    "received": "null",
    "path": [
      "environments",
      0,
      "os"
    ],
    "message": "Expected object, received null"
  }
]

I've tested my edits in https://github.com/spetrosi/infra.leapp/actions/runs/19170275013/job/54800766505

These changes are generated by Cursor AI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.56%. Comparing base (1d700fe) to head (9b055c8).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
- Coverage   91.58%   91.56%   -0.02%     
==========================================
  Files          14       14              
  Lines         867      865       -2     
  Branches      162      161       -1     
==========================================
- Hits          794      792       -2     
  Misses         73       73              

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[jamacku]

Thank you for the PR, but this is not right. You haven't updated the dist/ files, so you are still testing the original code.

But I know where the problem is.

[spetrosi]

It's shit code, sorry and thank you for looking at it.
Btw my successful run https://github.com/spetrosi/infra.leapp/actions/runs/19170275013/workflow#L199 used compose: null, not compose: omit as I thought, and It worked. So somewhat, my changes fix the issue.
My initial thought that null is a special value in yaml and it was not read as a string nor empty.