Fix bug in Merge When Checks Pass needing a pipeline

What does this MR do and why?

Merge When Checks Pass should function when a pipeline does not exist, and is not needed. Currently we inherit from MergeWhenPipelineSucceeds which requires a pipeline.

To test:

• Have a project which does not use pipelines and the setting 'pipeline required to pass' is not checked
• Ensure merge_when_checks_pass and additional_merge_when_checks_ready feature flags are on
• Be on master
• Create an MR that is mergeable except for an approval
• Click Auto Merge
• Click Approve
• It will fail to merge
• Switch to this branch
• Unapprove
• Cancel auto merge
• Click Auto merge
• Approve again
• It should merge fine

Related to: #441316 (closed)

ee/app/services/auto_merge/merge_when_checks_pass_service.rb

@@ -4,6 +4,17 @@ module AutoMerge
   class MergeWhenChecksPassService < AutoMerge::MergeWhenPipelineSucceedsService
     extend Gitlab::Utils::Override
 
+    override :process
+    def process(merge_request)
+      logger.info("Processing Automerge")
+
+      return unless merge_request.mergeable?
+
+      logger.info("Merge request mergeable")
+
+      merge_request.merge_async(merge_request.merge_user_id, merge_request.merge_params)
+    end
+
     override :overrideable_available_for_checks
     def overrideable_available_for_checks(merge_request)
       if Feature.enabled?(:additional_merge_when_checks_ready, merge_request.project)