CI: fix auto builds and make sure that we always have at least a single CI job

.github/workflows/ci.yml

@@ -77,14 +77,6 @@ jobs:
       matrix:
         # Check the `calculate_matrix` job to see how is the matrix defined.
         include: ${{ fromJSON(needs.calculate_matrix.outputs.jobs) }}
-    # GitHub Actions fails the workflow if an empty list of jobs is provided to
-    # the workflow, so we need to skip this job if nothing was produced by
-    # the Python script.
-    #
-    # Unfortunately checking whether a list is empty is not possible in a nice
-    # way due to GitHub Actions expressions limits.
-    # This hack is taken from https://github.com/ferrocene/ferrocene/blob/d43edc6b7697cf1719ec1c17c54904ab94825763/.github/workflows/release.yml#L75-L82
-    if: fromJSON(needs.calculate_matrix.outputs.jobs)[0] != null
     steps:
       - if: contains(matrix.os, 'windows')
         uses: msys2/setup-msys2@v2.22.0

src/ci/github-actions/calculate-job-matrix.py

@@ -136,7 +136,7 @@ def calculate_jobs(run_type: WorkflowRunType, job_data: Dict[str, Any]) -> List[
                 raise Exception(f"Custom job(s) `{unknown_jobs}` not found in auto jobs")
 
         return add_base_env(name_jobs(jobs, "try"), job_data["envs"]["try"])
-    elif run_type is AutoRunType:
+    elif isinstance(run_type, AutoRunType):
         return add_base_env(name_jobs(job_data["auto"], "auto"), job_data["envs"]["auto"])
 
     return []
@@ -192,6 +192,10 @@ if __name__ == "__main__":
     if run_type is not None:
         jobs = calculate_jobs(run_type, data)
     jobs = skip_jobs(jobs, channel)
+
+    if not jobs:
+        raise Exception("Scheduled job list is empty, this is an error")
+
     run_type = format_run_type(run_type)
 
     logging.info(f"Output:\n{yaml.dump(dict(jobs=jobs, run_type=run_type), indent=4)}")