github: don't upload to CodeCov if token unset

Author: vrutkovs

.github/workflows/rust.yml

@@ -86,6 +86,7 @@ jobs:
           token: ${{ secrets.CODECOV_TOKEN }}
           files: ./lcov.info
           fail_ci_if_error: true
+        if: ${{ secrets.CODECOV_TOKEN != '' }}
 
   lint:
     name: Lint