Enable optional arguments in findWindow

Both arguments of `FindWindow` can be optional by setting them to `nullptr` (http://msdn.microsoft.com/en-us/library/windows/desktop/ms633499%28v=vs.85%29.aspx); A thing which is _not_ possible at the moment.
As passing an empty string is useless, empty strings are passed as `nullptr` instead.
Other ways to fix this are welcomed. :)