Cannot read property 'items' of undefined on order query

[michaelbromley]

Describe the bug
When querying an Order with no items, Admin API returns:

[
  {
    "message": "Cannot read property 'items' of undefined",
    "locations": [
      {
        "line": 105,
        "column": 3
      }
    ],
    "path": [
      "order",
      "fulfillments"
    ]
  }
]

To Reproduce
Steps to reproduce the behavior:

1. In storefront/Shop API add an item to a new order
2. Remove that item from the order
3. In the Admin UI try to view that order detail page
4. See error

Expected behavior
No error

Environment (please complete the following information):

• @vendure/core version: 0.18.1
• Nodejs version: 12.4
• Database (mysql/postgres etc): mariadb