Removes extra Secret-Detection job when opening a merge request

changelogs/unreleased/secret-detection-remove-extra-job.yml

+---
+title: Remove extra Secret-Detection job on merge requests
+merge_request: 36884
+author:
+type: fixed

lib/gitlab/ci/templates/Security/Secret-Detection.gitlab-ci.yml

@@ -31,7 +31,7 @@ secret_detection:
   rules:
     - if: $SECRET_DETECTION_DISABLED
       when: never
-    - if: $CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH &&
+    - if: $CI_COMMIT_BRANCH && $CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH &&
           $GITLAB_FEATURES =~ /\bsecret_detection\b/
   script:
     - git fetch origin $CI_DEFAULT_BRANCH $CI_BUILD_REF_NAME