Fix rendering pipelines for a new merge request

What does this MR do?

This MR fixes a bug with invalid pipelines API call, which leads to raising an exception.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Added tests for this feature/bug

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8844#note_22930365

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -33,11 +33,17 @@
       end
 
       context 'when rendering JSON response' do
+        before do
+          create(:ci_pipeline, sha: fork_project.commit('remove-submodule').id,
+                               ref: 'remove-submodule',
+                               project: fork_project)
+        end
+
         it 'renders JSON including serialized pipelines' do
           submit_new_merge_request(format: :json)
 
-          expect(json_response).to have_key('pipelines')
           expect(response).to be_ok
+          expect(json_response).not_to be_empty
         end
       end
     end