sagemathgh-36348: .github/workflows/build.yml, doc-build.yml: Fix get…

…_ci_fixes

    
... for the case when there are no blocker PRs to be applied

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URL: sagemath#36348
Reported by: Matthias Köppe
Reviewer(s): Dima Pasechnik, Kwankyu Lee, Matthias Köppe, Tobias Diez