Fix calculating the default layout name

In the latest commits, the Strapper renderer automatically adds the `layout` hidden input field to the form.
90% of the time this field is empty, sadly in the controller we are checking if `$this->layout` is null.

This means that in browse views that use XML forms, when the user tries to add a new record he gets a not-so-nice white screen since FOF is trying to load the default layout.