Update nested_redords.avro to support nullable records

[sarutak]

This PR proposes to update nested_recods.avro to support nullable records.
This change is necessary for this PR.

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}]}

[sarutak]

@Samrose-Ahmed
Is this data OK for writing tests for that PR?

[Samrose-Ahmed]

Yes it is OK

[alamb]

Thank you @sarutak