allow processing of optional enum values

[DaanRademaker]

There was in issue when avro schema's were defined with an optional enum e.g.

        {
            "name": "Test",
            "type": "record",
            "fields": [
                {
                    "name": "c1",
                    "type": [
                        "null",
                        {
                            "type": "enum",
                            "symbols": ["passed", "failed"],
                            "name": "Status",
                        },
                    ],
                },
            ],
        }

[dan1elt0m]

LGTM! Thanks