error `unsupported file format <nil>` when a yaml value is empty

[LemonNeko]

Here is minimum reproduction code, runs on version 1.22.1:
main.go

package main

import (
	"github.com/nicksnyder/go-i18n/v2/i18n"
	"golang.org/x/text/language"
	"gopkg.in/yaml.v3"
)

func main() {
	bundle := i18n.NewBundle(language.English)
	bundle.RegisterUnmarshalFunc("yaml", yaml.Unmarshal)
	_, err := bundle.LoadMessageFile("en.yaml")
	if err != nil {
		panic(err)
	}

	localizer := i18n.NewLocalizer(bundle, language.English.String())
	localizedStr := localizer.MustLocalize(&i18n.LocalizeConfig{
		MessageID: "empty-key",
	})

	println(localizedStr)
}

en.yaml

key: value
empty-key-but-type-specified: ''
empty-key:

The yaml.Unmarshal function will return nil value when a key with empty value, and nil won't be matched in the code:

go-i18n/i18n/parse.go

Lines 63 to 117 in 521f196

 switch data := raw.(type) { 
 case string: 
 	if isInitialCall { 
 		return nil, errInvalidTranslationFile 
 	} 
 	m, err := NewMessage(data) 
 	return []*Message{m}, err 
  
 case map[string]interface{}: 
 	if isMapMessage { 
 		m, err := NewMessage(data) 
 		return []*Message{m}, err 
 	} 
 	messages = make([]*Message, 0, len(data)) 
 	for id, data := range data { 
 		// recursively scan map items 
 		messages, err = addChildMessages(id, data, messages) 
 		if err != nil { 
 			return nil, err 
 		} 
 	} 
  
 case map[interface{}]interface{}: 
 	if isMapMessage { 
 		m, err := NewMessage(data) 
 		return []*Message{m}, err 
 	} 
 	messages = make([]*Message, 0, len(data)) 
 	for id, data := range data { 
 		strid, ok := id.(string) 
 		if !ok { 
 			return nil, fmt.Errorf("expected key to be string but got %#v", id) 
 		} 
 		// recursively scan map items 
 		messages, err = addChildMessages(strid, data, messages) 
 		if err != nil { 
 			return nil, err 
 		} 
 	} 
  
 case []interface{}: 
 	// Backward compatibility for v1 file format. 
 	messages = make([]*Message, 0, len(data)) 
 	for _, data := range data { 
 		// recursively scan slice items 
 		childMessages, err := recGetMessages(data, isMessage(data), false) 
 		if err != nil { 
 			return nil, err 
 		} 
 		messages = append(messages, childMessages...) 
 	} 
  
 default: 
 	return nil, fmt.Errorf("unsupported file format %T", raw) 
 } 

Should we fallback to empty string "" when we got nil value?

[Nick Snyder]

Yeah that probably makes sense. I would accept a PR that adds a test and fixes this.