Use nil storage.ID for empty directories, emit in JSON

storage/tree.go

@@ -18,7 +18,7 @@ type Node struct {
 	ModTime time.Time   `json:"mtime"`
 	User    uint32      `json:"user"`
 	Group   uint32      `json:"group"`
-	Content ID          `json:"content"`
+	Content ID          `json:"content,omitempty"`
 }
 
 func NewTree() *Tree {

storagetest/main.go

@@ -36,6 +36,11 @@ func archive_dir(repo storage.Repository, path string) (storage.ID, error) {
 		return nil, err
 	}
 
+	// use nil ID for empty directories
+	if len(entries) == 0 {
+		return nil, nil
+	}
+
 	t := storage.NewTree()
 	for _, e := range entries {
 		node := storage.NodeFromFileInfo(e)