Make iterations finder ordering deterministic

ee/app/finders/iterations_finder.rb

@@ -82,8 +82,7 @@ class IterationsFinder
 
   # rubocop: disable CodeReuse/ActiveRecord
   def order(items)
-    order_statement = Gitlab::Database.nulls_last_order('due_date', 'ASC')
-    items.reorder(order_statement).order(:title)
+    items.reorder(:due_date).order(:title, { id: :asc })
   end
   # rubocop: enable CodeReuse/ActiveRecord
 

ee/spec/finders/iterations_finder_spec.rb

@@ -66,8 +66,8 @@ RSpec.describe IterationsFinder do
         expect(subject).to contain_exactly(root_closed_iteration, root_group_iteration, closed_iteration, started_group_iteration, upcoming_group_iteration)
       end
 
-      it 'orders iterations by due date' do
-        expect(subject.to_a).to eq([closed_iteration, root_closed_iteration, started_group_iteration, root_group_iteration, upcoming_group_iteration])
+      it 'orders iterations by due date and title' do
+        expect(subject.to_a).to eq([closed_iteration, root_closed_iteration, started_group_iteration, root_group_iteration, upcoming_group_iteration].sort_by { |a| [a.due_date, a.title, a.id] })
       end
     end