[TASK] Make $controllerName an optional argument

[mneuhaus]

We should change the signature of the resolveAvailableTemplateFiles method to allow $controllerName to be empty. We already support a null value for that argument, but keep that method signature as is for backwards compatability. We will change that signature towards a 2.0. https://github.com/NamelessCoder/Fluid/blob/master/src/View/TemplatePaths.php#L279