[Xwt.Mac] Fix TextEntryBackend to accept null Text value
[Xwt.Mac] Fix TextEntryBackend to accept null Text value

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