test pttodo.NewRootFromFile accepts both []Todo and Root

pttodo/root.go

@@ -1,6 +1,7 @@
 package pttodo
 
 import (
+	"bytes"
 	"os"
 
 	yaml "gopkg.in/yaml.v2"
@@ -13,6 +14,10 @@ type Root struct {
 }
 
 func NewRootFromFile(p string) (Root, error) {
+	if b, err := os.ReadFile(p); err == nil && len(bytes.TrimSpace(b)) == 0 {
+		return Root{}, nil
+	}
+
 	f, err := os.Open(p)
 	if os.IsNotExist(err) {
 		return Root{}, nil
@@ -24,7 +29,11 @@ func NewRootFromFile(p string) (Root, error) {
 
 	var result Root
 	if err := yaml.NewDecoder(f).Decode(&result); err != nil {
-		return Root{}, err
+		todos, err2 := NewTodosFromFile(p)
+		if err2 != nil {
+			return Root{}, err
+		}
+		result.Todo = todos
 	}
 
 	result.AutoMove()