Fix "Empty or null file name(s)" warning text

Remove '(s)' for methods taking only one path argument. The '(s)' has
been dropped in other places already ...

Change-Id: I9dc0d1cfa1e02f60bce901a309835f4bbfadde6d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>