Unique nullable field can't have multiple null values

[liwzhang]

Magql throws an error when you try to add more than 1 object with field = null where the field is a unique, nullable field.

  "data": null,
  "errors": [
    {
      "message": "magql argument validation",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "testing_create"
      ],
      "extensions": {
        "ctn": [
          "Testing with this test_field already exists."
        ]
      }
    }
  ]
}