创建订阅时，如果用户未输入promotion_code，则代码将失败并显示错误：

You passed an empty string for 'promotion_code'. We assume empty values are an attempt to unset a parameter; however 'promotion_code' cannot be unset. You should remove 'promotion_code' from your request or supply a non-empty value.

promo是从前端获取的（否则为空值）。我尝试使用一个if else语句 Package promotion_code，但也失败了-我如何向this传递null或空白值，或者如果没有输入促销代码，则省略此行？

const subscription = await stripe.subscriptions.create({
                customer: customer.id,
                items: [{ price: "price_1N1TKrGxUje7SlyIeNldQvLT" }],
                payment_settings: {
                  payment_method_options: {
                    card: {
                      request_three_d_secure: "any",
                    },
                  },
                  payment_method_types: ["card"],
                  save_default_payment_method: "on_subscription",
                },
                promotion_code: promo,
                expand: ["latest_invoice.payment_intent"],
              });