Fix scenario when no shipment available

I though that once the shipping method was set it's available on the
order, but apparently it's not always the case. At least some of the
test scenario have order with no shipment, thus no shipping method set.

app/controllers/split_checkout_controller.rb

@@ -112,7 +112,7 @@ class SplitCheckoutController < ::BaseController
   def recalculate_voucher
     return if @order.voucher_adjustments.empty?
 
-    return if @order.shipment.shipping_method.id == params[:shipping_method_id].to_i
+    return if @order.shipping_method&.id == params[:shipping_method_id].to_i
 
     VoucherAdjustmentsService.new(@order).update
   end

spec/controllers/split_checkout_controller_spec.rb

@@ -223,6 +223,20 @@ describe SplitCheckoutController, type: :controller do
 
               expect(response).to redirect_to checkout_step_path(:payment)
             end
+
+            context "when no shipments available" do
+              before do
+                order.shipments.destroy_all
+              end
+
+              it "recalculates the voucher adjustment" do
+                expect(service).to receive(:update)
+
+                put(:update, params:)
+
+                expect(response).to redirect_to checkout_step_path(:payment)
+              end
+            end
           end
         end
       end