dircache: allow empty string as root parent id

This was causing an internal error with the drime backend which has the
root parent id as an empty string. This shouldn't affect anything else.

lib/dircache/dircache.go

@@ -361,9 +361,6 @@ func (dc *DirCache) RootParentID(ctx context.Context, create bool) (ID string, e
 	} else if dc.rootID == dc.trueRootID {
 		return "", errors.New("is root directory")
 	}
-	if dc.rootParentID == "" {
-		return "", errors.New("internal error: didn't find rootParentID")
-	}
 	return dc.rootParentID, nil
 }