workflows: build-gluon: avoid failure when no path filter matches

The CI should be successful when there is nothing to check. Add if
condition as proposed in [1].

[1] https://github.com/dorny/paths-filter/issues/66#issuecomment-778267385

.github/workflows/build-gluon.yml

@@ -24,6 +24,7 @@ jobs:
 
   build_firmware:
     needs: changed
+    if: ${{ needs.changed.outputs.targets != '[]' && needs.changed.outputs.targets != '' }}
     strategy:
       fail-fast: false
       matrix: