fix: skip SonarQube scan when token is unavailable (#1575)

Fork PRs don't have access to repository secrets, causing the
SonarQube step to fail. Skip the step when SONAR_TOKEN is empty.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: daohoangson

.github/workflows/flutter.yml

@@ -46,5 +46,6 @@ jobs:
           path: "**/failures/"
 
       - uses: SonarSource/sonarqube-scan-action@299e4b793aaa83bf2aba7c9c14bedbb485688ec4 # v7.1.0
+        if: env.SONAR_TOKEN
         env:
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}