Avoids matrix.os-is-empty errors.

.github/workflows/main.yml

@@ -82,6 +82,7 @@ jobs:
   build-deb:
     name: Build DEB Packages
     needs: [check-upstream]
+    if: "matrix.os[0] != null"
     runs-on: ${{ matrix.os }}
 
     strategy:
@@ -156,6 +157,7 @@ jobs:
     needs: [check-upstream, build-deb]
     # Avoid push conflicts.
     concurrency: update_repository
+    if: "matrix.os[0] != null"
     runs-on: ${{ matrix.os }}
 
     strategy: