Fix: record MarshalJSON in case of default record with nullable field

[redaLaanait]

I came across the following case where record MarshalJSON generates an Invalid schema, which avro.Parse fails to parse, returning the error:

invalid default for field a. not a record

Schema:

{
  "type": "record",
  "name": "test",
  "namespace": "org.hamba.avro",
  "fields": [
    {
      "name": "a",
      "type": {
        "type": "record",
        "name": "test2",
        "fields": [
          {
            "name": "b",
            "type": "int"
          },
          {
            "name": "c",
            "type": [
              "null",
              "int"
            ]
          }
        ]
      },
      "default": {
        "b": 1,
        "c": null
      }
    }
  ]
}

The generated invalid schema:

{
  "name": "org.hamba.avro.test",
  "type": "record",
  "fields": [
    {
      "name": "a",
      "type": {
        "name": "org.hamba.avro.test2",
        "type": "record",
        "fields": [
          {
            "name": "b",
            "type": "int"
          },
          {
            "name": "c",
            "type": [
              "null",
              "int"
            ]
          }
        ]
      },
      "default": {
        "b": 1,
        "c": {}
      }
    }
  ]
}

The default value of c should be null instead of an empty JSON object.

If I got it right, a fix could be to define a custom MarshalJSON for the nullDefault value:

type nullDefaultType struct{}

func (nullDefaultType) MarshalJSON() ([]byte, error) {
	return []byte("null"), nil
}

var nullDefault nullDefaultType = struct{}{}

[nrwiersma]

LGTM