ci: skip sonar check if no Sonar token is provided

This prevents running (and failing) this task on master branch of forks.

Author: BlasiusSecundus

.github/workflows/ci.yml

@@ -96,4 +96,5 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}  # Needed to get PR information, if any
           SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}
+        if: env.SONAR_TOKEN != null
         run: ./gradlew build jacocoTestReport sonarqube --info