XmlView: check _serialize keys before using them
XmlView: check _serialize keys before using them

Before adding a view var to `$data`, make sure it exists.

this fixes #7067
cakephp/cakephp,ionas/cakephp,tenkoma/cakephp,Phillaf/cakephp,michaelze/cakephp,cakephp-fr/cakephp,CVO-Technologies/cakephp,erikfig/cakephp,jeffblack360/cakephp,pbaire/cakephp,timhoeppner/cakephp,timhoeppner/cakephp,michaelze/cakephp,jerems02/cakephp,tgr0ss/cakephp,davidyell/cakephp,tenkoma/cakephp,jeffblack360/cakephp,akintoey/cakephp,CakeDC/cakephp,tgr0ss/cakephp,dakota/cakephp,nrother/cakephp,Balachan27/cakephp,ndm2/cakephp,timhoeppner/cakephp,khoimt/cakephp-study,dkarvounaris/cakephp,jjj117/cakephp,dakota/cakephp,jeffblack360/cakephp,HavokInspiration/cakephp,ionas/cakephp,mvdriel/cakephp,ndm2/cakephp,pbaire/cakephp,bravo-kernel/cakephp,garethellis36/cakephp,akintoey/cakephp,petteyg/cakephp,jjj117/cakephp,elboletaire/cakephp,yinhe007/cakephp,thinkingmedia/cakephp,dkarvounaris/cakephp,eboominathan/cakephp,garethellis36/cakephp,CakeDC/cakephp,eryw/cakephp,cakephp/cakephp,ndm2/cakephp,adopico/cakephp,justinyost/cakephp,CakeDC/cakephp,tgr0ss/cakephp,tisu2tisu/cakephp,pbaire/cakephp,yinhe007/cakephp,eryw/cakephp,petteyg/cakephp,tenkoma/cakephp,Phillaf/cakephp,burriko/cakephp,mvdriel/cakephp,akintoey/cakephp,yinhe007/cakephp,HavokInspiration/cakephp,CVO-Technologies/cakephp,eryw/cakephp,dakota/cakephp,cakephp-fr/cakephp,davidyell/cakephp,jjj117/cakephp,davidyell/cakephp,berrygoudswaard/cakephp,bravo-kernel/cakephp,tisu2tisu/cakephp,michaelze/cakephp,ADmad/cakephp,kumamidori/cakephp,web-player/cakephp,ndm2/cakephp,bravo-kernel/cakephp,petteyg/cakephp,mvdriel/cakephp,jerems02/cakephp,kumamidori/cakephp,cakephp/cakephp,justinyost/cakephp,thinkingmedia/cakephp,berrygoudswaard/cakephp,petteyg/cakephp,nrother/cakephp,justinyost/cakephp,dakota/cakephp,burriko/cakephp,erikfig/cakephp,eboominathan/cakephp,khoimt/cakephp-study,burriko/cakephp,kumamidori/cakephp,Phillaf/cakephp,elboletaire/cakephp,berrygoudswaard/cakephp,ionas/cakephp,nrother/cakephp,cakephp-fr/cakephp,gmponos/cakephp,CVO-Technologies/cakephp,dkarvounaris/cakephp,erikfig/cakephp,cakephp/cakephp,Balachan27/cakephp,ADmad/cakephp,adopico/cakephp,gmponos/cakephp,ADmad/cakephp,elboletaire/cakephp,eboominathan/cakephp,mvdriel/cakephp,adopico/cakephp,garethellis36/cakephp,jerems02/cakephp,tenkoma/cakephp,Balachan27/cakephp,tisu2tisu/cakephp,gmponos/cakephp,web-player/cakephp,HavokInspiration/cakephp,garethellis36/cakephp,CakeDC/cakephp,web-player/cakephp,ADmad/cakephp,thinkingmedia/cakephp,khoimt/cakephp-study