Can't decode nested field which parent field could be null or record

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I have next code:

```go
package main

import (
"log"

"github.com/linkedin/goavro/v2"
)

const schema = `{
"type": "record",
"name": "Payload",
"fields": [
{
"name": "image",
"default": "null",
"type": [
"null",
{
"type": "record",
"name": "Image",
"fields": [
{
"name": "url",
"type": "string"
},
{
"name": "caption",
"type": "string",
"default": "undefined"
}
]
}
]
}
]
}

`

func main() {
codec, err := goavro.NewCodec(schema)
if err != nil {
println("schema is wrong")
log.Fatalln(err)
}

str := `{"image": {"caption": "asd"}}`

a, b, err := codec.NativeFromTextual([]byte(str))

if err != nil {
println(err.Error())
} else {
println(string(b))
println(a)
}

}
```

Image is optional field: it could be null or contain some fields (url and caption).

Instead of successful parsing of JSON i receving next error message: `cannot decode textual record "SendPayload": cannot decode textual union: cannot decode textual map: cannot determine codec: "caption" for key: "image"`.

Problem is that avro schema contains `image.caption` field. I tried sending image with caption, url and both, but problem persists.

Start with the provided schema and reproduction, then trace codec.NativeFromTextual while it decodes the optional image union and nested record. Confirm the behavior for image as null, with url, with caption, and with both; done means the valid JSON forms decode successfully without the reported union/map error.

Written by the indexing model from the issue text.