Describe the bug

We are testing on order UI and found out that when apply coupon in draft order it's working but when we try to remove it, it shows error toast. In the docs, remove coupon code must use string but the coupon code in body is null.

To Reproduce
Steps to reproduce the behavior:

1. Go to promotion, create new promotion with discount % and code ex: 'TEST'
2. Go to order, create new draft order
3. Add any new products
4. Apply coupon code 'TEST'
5. Then click remove on dropdown select
6. See error

Expected behavior
It should be able to add and remove coupon in draft order.

Environment (please complete the following information):

• @vendure/core version: 3.0.0
• Nodejs version: 20
• Database (mysql): 8

Additional context
N/A