Fix final_delay exception in MigrationHelpers

lib/gitlab/database/migration_helpers.rb

@@ -1090,7 +1090,7 @@ into similar problems in the future (e.g. when new tables are created).
           delay_interval = BackgroundMigrationWorker.minimum_interval
         end
 
-        final_delay = nil
+        final_delay = 0
 
         model_class.each_batch(of: batch_size) do |relation, index|
           start_id, end_id = relation.pluck(Arel.sql('MIN(id), MAX(id)')).first

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -1373,6 +1373,14 @@ describe Gitlab::Database::MigrationHelpers do
         end
       end
 
+      it 'returns zero when nothing gets queued' do
+        Sidekiq::Testing.fake! do
+          final_delay = model.queue_background_migration_jobs_by_range_at_intervals(User.none, 'FooJob', 10.minutes)
+
+          expect(final_delay).to eq(0)
+        end
+      end
+
       context 'with batch_size option' do
         it 'queues jobs correctly' do
           Sidekiq::Testing.fake! do