Add check for non empty value for View::$layout.
Add check for non empty value for View::$layout.

This helps provide verbose message for those upgrading from 3.x.

cakephp/cakephp,cakephp/cakephp,dakota/cakephp,ndm2/cakephp,CakeDC/cakephp,ADmad/cakephp,CakeDC/cakephp,CakeDC/cakephp,ADmad/cakephp,cakephp/cakephp,ADmad/cakephp,ndm2/cakephp,CakeDC/cakephp,ndm2/cakephp,dakota/cakephp,ndm2/cakephp,dakota/cakephp,ADmad/cakephp,dakota/cakephp,cakephp/cakephp