Add test to cover allowEmptyString with when callback.
Add test to cover allowEmptyString with when callback.

Refs cakephp/docs#6307

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