Return default group MR approval settings

When there is no MR approval settings on group, we would like the UI to
render default and non-persistent settings. This is consistent with the
update endpoint where we create new settings when there is no existing
ones.

ee/lib/api/group_merge_request_approval_settings.rb

@@ -21,8 +21,9 @@ module API
         get do
           authorize! :admin_merge_request_approval_settings, user_group
 
-          present user_group.group_merge_request_approval_setting,
-            with: ::API::Entities::GroupMergeRequestApprovalSetting
+          setting = GroupMergeRequestApprovalSetting.find_or_initialize_by_group(user_group)
+
+          present setting, with: ::API::Entities::GroupMergeRequestApprovalSetting
         end
 
         desc 'Update existing merge request approval setting' do

ee/spec/requests/api/group_merge_request_approval_settings_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe API::GroupMergeRequestApprovalSettings do
-  let_it_be(:group) { create(:group) }
+  let_it_be_with_reload(:group) { create(:group) }
   let_it_be(:user) { create(:user) }
   let_it_be(:setting) { create(:group_merge_request_approval_setting, group: group) }
 
@@ -47,6 +47,19 @@ RSpec.describe API::GroupMergeRequestApprovalSettings do
 
           expect(response).to match_response_schema('public_api/v4/group_merge_request_approval_settings', dir: 'ee')
         end
+
+        context 'when the group does not have existing settings' do
+          before do
+            group.group_merge_request_approval_setting.delete
+          end
+
+          it 'returns in-memory default settings', :aggregate_failures do
+            get api(url, user)
+
+            expect(response).to have_gitlab_http_status(:ok)
+            expect(json_response['allow_author_approval']).to eq(false)
+          end
+        end
       end
 
       context 'when the user is not authorised' do