Update nested_redords.avro to support nullable records (#94)

This PR proposes to update `nested_recods.avro` to support nullable
records.
This change is necessary for [this
PR](apache/datafusion#7663).

This change appends new fields `f3` and `f4` to the existing schema (the
last two fields).
`f3` is nullable record, and `f4` is array of nullable record.
```
{
    "name": "record1",
    "namespace": "ns1",
    "type": "record",
    "fields": [
        {
            "name": "f1",
            "type": {
                "name": "record2",
                "namespace": "ns2",
                "type": "record",
                "fields": [
                    {
                        "name": "f1_1",
                        "type": "string"
                    },  {
                        "name": "f1_2",
                        "type": "int"
                    },  {
                        "name": "f1_3",
                        "type": {
                            "name": "record3",
                            "namespace": "ns3",
                            "type": "record",
                            "fields": [
                                {
                                    "name": "f1_3_1",
                                    "type": "double"
                                }
                            ]
                        }
                    }
                ]
            }
        },  {
            "name": "f2",
            "type": "array",
            "items": {
                "name": "record4",
                "namespace": "ns4",
                "type": "record",
                "fields": [
                    {
                        "name": "f2_1",
                        "type": "boolean"
                    },  {
                        "name": "f2_2",
                        "type": "float"
                    }
                ]
            }
        },  {
            "name": "f3",
            "type": [
                "null",
                {
                    "name": "record5",
                    "namespace": "ns5",
                    "type": "record",
                    "fields": [
                        {
                            "name": "f3_1",
                            "type": "string"
                        }
                    ]
                }
            ],
            "default": null
        },  {
            "name": "f4",
            "type": "array",
            "items": [
                "null",
                {
                    "name": "record6",
                    "namespace": "ns6",
                    "type": "record",
                    "fields": [
                        {
                            "name": "f4_1",
                            "type": "long"
                        }
                    ]
                }
            ]
        }
    ]
}
```

And the data represented in JSON is as follows.
```
{"f1":{"f1_1":"aaa","f1_2":10,"f1_3":{"f1_3_1":3.14}},"f2":[{"f2_1":true,"f2_2":1.2000000476837158},{"f2_1":true,"f2_2":2.200000047683716}],"f3":{"f3_1":"xyz"},"f4":[{"f4_1":200},null]}
{"f1":{"f1_1":"bbb","f1_2":20,"f1_3":{"f1_3_1":3.14}},"f2":[{"f2_1":false,"f2_2":10.199999809265137}],"f3":null,"f4":[null,{"f4_1":300}]}
```

Author: alamb