cache: fix nil pointer deref when using lsjson on cached directory

This stops embedding the fs.Directory into the cache.Directory because
it can be nil and implements an ID method which checks the nil status.

See: https://forum.rclone.org/t/runtime-error-with-cache-remote-and-lsjson/6305

backend/cache/directory.go

@@ -12,7 +12,7 @@ import (
 
 // Directory is a generic dir that stores basic information about it
 type Directory struct {
-	fs.Directory `json:"-"`
+	Directory fs.Directory `json:"-"` // can be nil
 
 	CacheFs      *Fs    `json:"-"`       // cache fs
 	Name         string `json:"name"`    // name of the directory
@@ -125,6 +125,14 @@ func (d *Directory) Items() int64 {
 	return d.CacheItems
 }
 
+// ID returns the ID of the cached directory if known
+func (d *Directory) ID() string {
+	if d.Directory == nil {
+		return ""
+	}
+	return d.Directory.ID()
+}
+
 var (
 	_ fs.Directory = (*Directory)(nil)
 )