Fix error added when resetting order flow on an empty order

#4369 introduced a regression
where when calling `Spree::Order#restart_checkout_flow` on an empty
order, an error would be added to the order. That happened when calling
`#next` and the validation failed because no line items were present.

We restore the previous behavior where we only try going to the
`"address"` state if the order has line items.

core/app/models/spree/order.rb

@@ -522,7 +522,7 @@ def restart_checkout_flow
         state: 'cart',
         updated_at: Time.current
       )
-      self.next
+      self.next if line_items.any?
     end
 
     def refresh_shipment_rates

core/spec/models/spree/order_spec.rb

@@ -594,6 +594,14 @@ def merge!(other_order, user = nil)
         order = create(:order, state: "delivery")
         expect{ order.restart_checkout_flow }.to change{ order.state }.from("delivery").to("cart")
       end
+
+      it "doesn't add errors to the order" do
+        order = create(:order, state: "delivery", line_items: [])
+
+        order.restart_checkout_flow
+
+        expect(order.errors).to be_empty
+      end
     end
   end