Add tests for empty expression objects in association conditions.
Add tests for empty expression objects in association conditions.

Empty expression objects currently cause invalid SQL when added to
a query. I feel they should be no-ops instead.

Refs #6568

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