Decoder overrides defaults with null values

[yhabteab]

Is your feature request related to a problem? Please describe.

The decoder overrides the default values of a nested struct with zero-initialised value.

package main

import (
    "fmt"
    //"gopkg.in/yaml.v3"
    "strings"
    "github.com/goccy/go-yaml"
)

type Default struct {
    Key string `yaml:"key"`
}

type Test struct {
    First    string  `yaml:"first"`
    Defaults Default `yaml:"second"`
}

func main() {
    yml := `
first: "Test"
second:
  # Just want to document the fields of the nested struct
#  key: "Value from YAML"
`

    test := Test{Defaults: Default{Key: "My default Value"}}
    d := yaml.NewDecoder(strings.NewReader(yml))
    if err := d.Decode(&test); err != nil {
        fmt.Println("cannot decode: ", err)
        return
    }

    fmt.Printf("%#v", test)
}

This prints with go-yaml main.Test{First:"Test", Defaults:main.Default{Key:""}} while with.. yaml.v3 it prints main.Test{First:"Test", Defaults:main.Default{Key:"My default Value"}}.

Describe the solution you'd like

It might be desirable to stick with the current behaviour by default (as it might break some applications that depend on it, I guess!), however, it would be nice if there would be an option to control this, something like yaml.IgnoreNullValues().