Expose merge request flag as boolean in the pipeline entity

Changelog: fixed

app/models/ci/pipeline.rb

@@ -1166,7 +1166,7 @@ module Ci
 
     def merge_request?
       if Feature.enabled?(:ci_pipeline_merge_request_presence_check, default_enabled: :yaml)
-        merge_request_id.present? && merge_request
+        merge_request_id.present? && merge_request.present?
       else
         merge_request_id.present?
       end

spec/serializers/ci/pipeline_entity_spec.rb

@@ -224,7 +224,8 @@ RSpec.describe Ci::PipelineEntity do
         end
 
         it 'makes atached flag true' do
-          expect(subject[:flags][:merge_request_pipeline]).to be_truthy
+          expect(subject[:flags][:merge_request_pipeline]).to be true
+          expect(subject[:flags][:merge_request]).to be true
         end
 
         it 'exposes source sha and target sha' do