fix(ci): skip Coveralls upload when token unavailable

The coverage workflow was failing on Dependabot PRs because
COVERALLS_REPO_TOKEN is not available to PRs from forks for security
reasons. Add a conditional check to skip the upload step when the
token is missing, while still running coverage generation.

This allows PRs to pass CI while retaining coverage reports on master.

.github/workflows/coverage.yaml

@@ -57,6 +57,7 @@ jobs:
         working-directory: pacman
 
       - name: Coveralls upload
+        if: ${{ secrets.COVERALLS_REPO_TOKEN != '' }}
         uses: coverallsapp/github-action@v2
         with:
           github-token: ${{ secrets.COVERALLS_REPO_TOKEN }}