Ensure VSM stage has relative position

Ensure that all VSM stage has relative position before moving a stage.
This fixes a problem where moving a stage after another stage (where the
relative position was null) sets the position incorrectly.

ee/app/services/analytics/cycle_analytics/stages/update_service.rb

@@ -56,6 +56,9 @@ module Analytics
         # rubocop: enable CodeReuse/ActiveRecord
 
         def handle_position_change
+          # make sure that unpositioned items have relative_position
+          @stage.class.move_nulls_to_end(parent.cycle_analytics_stages.ordered)
+
           move_before_id = params.delete(:move_before_id)
           move_after_id = params.delete(:move_after_id)
 

ee/spec/services/analytics/cycle_analytics/stages/update_service_spec.rb

@@ -119,6 +119,22 @@ describe Analytics::CycleAnalytics::Stages::UpdateService do
     let!(:middle_stage) { create(:cycle_analytics_group_stage, group: group, relative_position: 11) }
     let!(:last_stage) { create(:cycle_analytics_group_stage, group: group, relative_position: 12) }
 
+    context 'when there are stages without position' do
+      let!(:unpositioned_stage1) { create(:cycle_analytics_group_stage, group: group) }
+      let!(:unpositioned_stage2) { create(:cycle_analytics_group_stage, group: group) }
+
+      before do
+        params[:id] = first_stage.id
+        params[:move_after_id] = unpositioned_stage2.id
+      end
+
+      it 'when moving the stage down' do
+        subject
+
+        expect(persisted_stages.last(3)).to eq([unpositioned_stage1, unpositioned_stage2, first_stage])
+      end
+    end
+
     context 'when moving the stage down' do
       before do
         params[:id] = first_stage.id