ci: skip coveralls if no coverdata found

.github/workflows/run_test_cases.yaml

@@ -146,6 +146,7 @@ jobs:
       - name: send to coveralls
         if: github.repository == 'emqx/emqx'
         run: |
+          stat -t -- './_build/test/cover/*.coverdata' >/dev/null 2>&1 || exit 0
           ./scripts/ct/run.sh --ci --app ${{ matrix.app }} --only-up
           docker exec \
           -e IS_CI="$IS_CI" \