[Xwt.Mac] Fix Label backend to accept null Text value
[Xwt.Mac] Fix Label backend to accept null Text value

steffenWi/xwt,mono/xwt,hwthomas/xwt,lytico/xwt,iainx/xwt,mminns/xwt,cra0zy/xwt,directhex/xwt,antmicro/xwt,sevoku/xwt,residuum/xwt,hamekoz/xwt,akrisiun/xwt,mminns/xwt,TheBrainTech/xwt