[codecov] Conditionally triggering the step if secret is present

.github/workflows/rust.yml

@@ -60,6 +60,7 @@ jobs:
       - name: Test and Generate code coverage
         run: cargo +nightly llvm-cov --all-features --workspace --lcov --doctests --output-path lcov.info
       - name: Codecov
+        if: ${{ secrets.CODECOV_TOKEN != '' }}
         uses: codecov/codecov-action@v3
         with:
           verbose: true