Stripe/NodeJS：为 promotion

创建订阅时，如果用户未输入促销代码，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 取自前端（否则为空值）。我尝试使用围绕 promotion_code 的 if else 语句也失败了 - 如果未输入促销代码，我如何将 null 或空白值传递给它，或者省略此行？

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"],
              });