Handle situation for viewRoot = null

With example from http://www.nfjsone.com/blog/ed_burns/2009/09/dealing_gracefully_with_viewexpiredexception_in_jsf2 and a coding typo like:
nav.handleNavigation(fc, null, "/nonExistentView.xhtml");

you get facesContext.viewRoot = null even in render_response phase

codi throw NPEs in such case, see attached patch for details.