Prevent clearing label background when it's set.
Prevent clearing label background when it's set.

I.e. prevent clearing `label.backgroundColor` when it's neither unset
nor white.

colemarkham/nui,jianwoo/nui,tombenner/nui,timbodeit/nui,KaterinaPetrova/nui,AaronYi/nui,Jake000/nui,colemarkham/nui,jianwoo/nui,ivanyang1984/nui,foxsofter/nui,tutoru/nui,Suninus/nui,mutualmobile/nui,jianwoo/nui,tutoru/nui,foxsofter/nui,Neosperience/nui,antobito11/nui,odigeoteam/nui,foxsofter/nui,ivanyang1984/nui,Stunner/nui,Jake000/nui,Jake000/nui,Neosperience/nui,TribeMedia/nui,AaronYi/nui,colemarkham/nui,jianwoo/nui,Joulebug/nui,tutoru/nui,TribeMedia/nui,ivanyang1984/nui,AaronYi/nui,Suninus/nui,AnyPresence/nui,peantunes/nui,Suninus/nui,TribeMedia/nui,Neosperience/nui