Decoder overrides defaults with null values

[Yonas Habteab]

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().