Avoid breaking pipeline for unconfigured SonarQube

As quality should be checked, a failing stage is introduced to keep
integrator aware.

.gitlab-ci.yml

@@ -27,7 +27,7 @@ verify:
   stage: verify
   script:
     - mvn $MAVEN_CLI_OPTS verify site
-    - mvn $MAVEN_CLI_OPTS sonar:sonar
+    - test -n "$SONAR_TOKEN" -a -n "$SONAR_PROJECT_KEY" -a "$SONAR_PROJECT_NAME" && mvn $MAVEN_CLI_OPTS sonar:sonar
           -Dsonar.login=${SONAR_TOKEN}
           -Dsonar.branch.name=${CI_COMMIT_REF_NAME}
           -Dsonar.projectKey="$SONAR_PROJECT_KEY"
@@ -40,6 +40,17 @@ verify:
       junit:
         - target/surefire-reports/*.xml
 
+verify:warning:
+  stage: verify
+  script:
+    - echo "Please, configure SonarQube!"
+    - exit 1
+  allow_failure: true
+  rules:
+    - if: $SONAR_TOKEN == null
+    - if: $SONAR_PROJECT_KEY == null
+    - if: $SONAR_PROJECT_NAME == null
+
 # Convert and import in GitLab coverage data.
 # The information is then available directly in merge-request view.
 # Cf. https://docs.gitlab.com/ee/user/project/merge_requests/test_coverage_visualization.html