[Mac] Fix ImageView.Image setter
[Mac] Fix ImageView.Image setter

It now acceppts null as value.
cra0zy/xwt,iainx/xwt,hamekoz/xwt,directhex/xwt,mminns/xwt,TheBrainTech/xwt,mminns/xwt,lytico/xwt,mono/xwt,hwthomas/xwt,steffenWi/xwt,sevoku/xwt,akrisiun/xwt,residuum/xwt,antmicro/xwt