Custom attributes of variable products cannot be queried.

[mlipscombe]

Describe the bug
Retrieving the attributes field of variable products that contain custom attributes returns NULL with the following error for any custom attributes:

{
	"errors": [{
		"debugMessage": "Cannot return null for non-nullable field ProductAttribute.attributeId.",
		"message": "Internal server error",
		"category": "internal",
		"locations": [{
			"line": 39,
			"column": 11
		}],
		"path": ["products", "nodes", 0, "attributes", "nodes", 2, "attributeId"]
	}]
}

To Reproduce
Steps to reproduce the behavior:

1. Create a product with custom attributes.
2. Attempt to query the attributes:

{
  productBy(id: "x") {
    attributes {
      nodes {
        attributeId
      }
    }
  }
}

Expected behavior
No error or warning.