ftp: fix listing of an empty root returning: error dir not found

Before this change if rclone listed an empty root directory then it
would return an error dir not found.

After this change we assume the root directory exists and don't
attempt to check it which was failing before.

See: https://forum.rclone.org/t/ftp-empty-directory-yields-directory-not-found-error/12069/

backend/ftp/ftp.go

@@ -299,6 +299,14 @@ func translateErrorDir(err error) error {
 func (f *Fs) findItem(remote string) (entry *ftp.Entry, err error) {
 	// defer fs.Trace(remote, "")("o=%v, err=%v", &o, &err)
 	fullPath := path.Join(f.root, remote)
+	if fullPath == "" || fullPath == "." || fullPath == "/" {
+		// if root, assume exists and synthesize an entry
+		return &ftp.Entry{
+			Name: "",
+			Type: ftp.EntryTypeFolder,
+			Time: time.Now(),
+		}, nil
+	}
 	dir := path.Dir(fullPath)
 	base := path.Base(fullPath)
 
@@ -366,7 +374,7 @@ func (f *Fs) dirExists(remote string) (exists bool, err error) {
 // This should return ErrDirNotFound if the directory isn't
 // found.
 func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err error) {
-	// defer fs.Trace(dir, "curlevel=%d", curlevel)("")
+	// defer log.Trace(dir, "dir=%q", dir)("entries=%v, err=%v", &entries, &err)
 	c, err := f.getFtpConnection()
 	if err != nil {
 		return nil, errors.Wrap(err, "list")