Handle attributes with no inputFields or fields

[orta]

We have an enum with no fields, or inputFields in the schema.json

        {
          "kind": "UNION",
          "name": "PartnerTypes",
          "description": null,
          "fields": null,
          "inputFields": null,
          "interfaces": null,
          "enumValues": null,
          "possibleTypes": [
            {
              "kind": "OBJECT",
              "name": "Partner",
              "ofType": null
            },
            {
              "kind": "OBJECT",
              "name": "ExternalPartner",
              "ofType": null
            }
          ]
        },

This currently crashes the process. Will send a PR.