Fix empty job matrix

.github/workflows/ci.yml

@@ -63,6 +63,7 @@ jobs:
     strategy:
       matrix:
         include: "${{ fromJSON(needs.calculate_matrix.outputs.jobs) }}"
+    if: "fromJSON(needs.calculate_matrix.outputs.jobs)[0] != null"
     defaults:
       run:
         shell: "${{ contains(matrix.os, 'windows') && 'msys2 {0}' || 'bash' }}"

src/ci/github-actions/ci.yml

@@ -373,6 +373,14 @@ 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
 
   master:
     name: master