Error when calling adjustOrderLine with quantity 0

[michaelbromley]

Describe the bug
When executing the mutation adjustOrderLine and passing a quantity of 0, the server errors with "Cannot read property 'pendingAdjustments' of undefined"

To Reproduce
Steps to reproduce the behavior:

1. Add an item to the order in the Shop API
2. Execute adjustOrderLine(orderLineId: <ID>, quantity: 0) { ...}
3. See error:

{
"errors": [
  {
    "message": "Cannot read property 'pendingAdjustments' of undefined",
    "locations": [
      {
        "line": 2,
        "column": 3
      }
    ],
    "path": [
      "adjustOrderLine"
    ]
  }
],
"data": {
  "adjustOrderLine": null
}
}

Expected behavior
In this case it should behave the same as removeOrderLine.

Environment (please complete the following information):

• @vendure/core version: 0.14.0
• Nodejs version: 12.x
• Database (mysql/postgres etc): mysql