Strange behavior using goavro.Union with map. (Question)

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I'm stumbling with some strange behavior using Union and I don't understand if I'm missing something in using API or if it is a possible issue. Please check the following piece of code

Thanks for support

```
package main

import (
"fmt"

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

const schema = `{
"type": "record",
"name": "appPublished",
"namespace": "it.balyfix",
"fields": [{
"name": "appId",
"type": "string"
},
{
"name": "metaDataMap",
"type": [
"null",
{
"type": "map",
"values" : ["string", "int", "boolean", "long", "double"]
}
],
"default": null
}
]
}`

func main() {
codec, err := goavro.NewCodec(schema)
if err != nil {
fmt.Println(err)
}
attemptOneKO(map[string]interface{}{"appId": "app1"}, codec)
attemptTwoKO(map[string]interface{}{"appId": "app1"}, codec)
attemptWithoutMetadataOK(map[string]interface{}{"appId": "app1"}, codec)
}

func attemptOneKO(message2Send map[string]interface{}, codec *goavro.Codec) {
metaDataMap := make(map[string]string)
metaDataMap["key1"] = "value1"
metaDataMap["key2"] = "value2"
message2Send["metaDataMap"] = metaDataMap
buf, err := codec.TextualFromNative(nil, message2Send)
if err != nil {
fmt.Println(err)
}
fmt.Println(string(buf))

}

func attemptTwoKO(message2Send map[string]interface{}, codec *goavro.Codec) {

message2Send["metaDataMap"] = goavro.Union("string", map[string]interface{}{
"child1": "foo",
"child2": "bar"})
buf, err := codec.TextualFromNative(nil, message2Send)
if err != nil {
fmt.Println(err)
}
fmt.Println(string(buf))

}

func attemptWithoutMetadataOK(message2Send map[string]interface{}, codec *goavro.Codec) {
buf, err := codec.TextualFromNative(nil, message2Send)
if err != nil {
fmt.Println(err)
}
fmt.Println(string(buf))

}
```
I'm using v2.12.0

Start by reproducing the three attempts in the provided Go program with goavro v2.12.0, then inspect NewCodec, TextualFromNative, and Union handling for the map field. Compare the returned errors and output for each input, and document whether the behavior is expected or identify a focused failing case.

Written by the indexing model from the issue text.