github: don't upload to CodeCov if token unset

Author: vrutkovs

.github/workflows/rust.yml

@@ -47,6 +47,7 @@ jobs:
       PROJECT_NAME_UNDERSCORE: masto_thread_renderer
       CARGO_INCREMENTAL: 0
       RUSTFLAGS: -Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests
+      CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -86,6 +87,7 @@ jobs:
           token: ${{ secrets.CODECOV_TOKEN }}
           files: ./lcov.info
           fail_ci_if_error: true
+        if: ${{ env.CODECOV_TOKEN != '' }}
 
   lint:
     name: Lint