Trigger db testing only when token is present

.gitlab/ci/rails.gitlab-ci.yml

@@ -377,7 +377,7 @@ db:rollback:
     - bundle exec rake db:migrate SKIP_SCHEMA_VERSION_CHECK=true
 
 db:gitlabcom-database-testing:
-  extends: .rails:rules:ee-and-foss-mr-with-migration
+  extends: .rails:rules:db:gitlabcom-database-testing
   stage: test
   image: ruby:2.7-alpine
   needs: []

.gitlab/ci/rules.gitlab-ci.yml

@@ -523,6 +523,13 @@
       changes: *db-patterns
     - <<: *if-merge-request-title-run-all-rspec
 
+.rails:rules:db:gitlabcom-database-testing:
+  rules:
+    - if: '$GITLABCOM_DATABASE_TESTING_TRIGGER_TOKEN == null'
+      when: never
+    - <<: *if-merge-request
+      changes: *db-patterns
+
 .rails:rules:ee-and-foss-unit:
   rules:
     - changes: *backend-patterns