dropbox: save an API request when at the root

Before this change, rclone always emitted an API request to discover
what type of thing the root is.

This is unecessary as it is always a directory.

backend/dropbox/dropbox.go

@@ -566,6 +566,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 	f.setRoot(root)
 
 	// See if the root is actually an object
+	if f.root != "" {
 		_, err = f.getFileMetadata(ctx, f.slashRoot)
 		if err == nil {
 			newRoot := path.Dir(f.root)
@@ -576,6 +577,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 			// return an error with an fs which points to the parent
 			return f, fs.ErrorIsFile
 		}
+	}
 	return f, nil
 }