fix format with empty value (#654)

decode_test.go

@@ -3196,6 +3196,41 @@ foo:
 	}
 }
 
+func TestBytesUnmarshalerWithEmptyValue(t *testing.T) {
+	type T struct{}
+
+	unmarshaler := func(dst *T, b []byte) error {
+		var v any
+		return yaml.Unmarshal(b, &v)
+	}
+
+	yml := `
+map: &m {}
+seq: &seq []
+foo: # comment
+bar: *m
+baz: *seq
+`
+	m := yaml.CommentMap{}
+	var v T
+	if err := yaml.UnmarshalWithOptions(
+		[]byte(yml),
+		&v,
+		yaml.CommentToMap(m),
+		yaml.CustomUnmarshaler[T](unmarshaler),
+	); err != nil {
+		t.Fatal(err)
+	}
+	if err := yaml.UnmarshalWithOptions(
+		[]byte(yml),
+		&v,
+		yaml.CustomUnmarshaler[T](unmarshaler),
+	); err != nil {
+		t.Fatal(err)
+	}
+
+}
+
 func TestIssue650(t *testing.T) {
 	type Disk struct {
 		Name   string `yaml:"name"`

internal/format/format.go

@@ -284,6 +284,9 @@ func (f *Formatter) formatFile(file *ast.File) string {
 }
 
 func (f *Formatter) origin(tk *token.Token) string {
+	if tk == nil {
+		return ""
+	}
 	if f.existsComment {
 		return tk.Origin
 	}
@@ -349,10 +352,6 @@ func (f *Formatter) formatDirective(n *ast.DirectiveNode) string {
 }
 
 func (f *Formatter) formatMapping(n *ast.MappingNode) string {
-	if len(n.Values) == 0 {
-		return "{}"
-	}
-
 	var ret string
 	if n.IsFlowStyle {
 		ret = f.origin(n.Start)
@@ -379,10 +378,6 @@ func (f *Formatter) formatMappingKey(n *ast.MappingKeyNode) string {
 }
 
 func (f *Formatter) formatSequence(n *ast.SequenceNode) string {
-	if len(n.Values) == 0 {
-		return "[]"
-	}
-
 	var ret string
 	if n.IsFlowStyle {
 		ret = f.origin(n.Start)