Replace empty attributes list with `none`
Replace empty attributes list with `none`

Tmux-1.6 also thinks that empty value for window-status-…-attr options is
invalid

blindFS/powerline,lukw00/powerline,cyrixhero/powerline,s0undt3ch/powerline,dragon788/powerline,IvanAli/powerline,Luffin/powerline,russellb/powerline,QuLogic/powerline,EricSB/powerline,S0lll0s/powerline,Liangjianghao/powerline,DoctorJellyface/powerline,junix/powerline,S0lll0s/powerline,IvanAli/powerline,DoctorJellyface/powerline,darac/powerline,darac/powerline,xxxhycl2010/powerline,russellb/powerline,cyrixhero/powerline,bartvm/powerline,areteix/powerline,lukw00/powerline,QuLogic/powerline,s0undt3ch/powerline,prvnkumar/powerline,seanfisk/powerline,EricSB/powerline,kenrachynski/powerline,prvnkumar/powerline,bezhermoso/powerline,lukw00/powerline,xxxhycl2010/powerline,bartvm/powerline,Luffin/powerline,EricSB/powerline,kenrachynski/powerline,blindFS/powerline,bartvm/powerline,bezhermoso/powerline,seanfisk/powerline,xfumihiro/powerline,bezhermoso/powerline,dragon788/powerline,xxxhycl2010/powerline,areteix/powerline,seanfisk/powerline,darac/powerline,kenrachynski/powerline,xfumihiro/powerline,Liangjianghao/powerline,cyrixhero/powerline,IvanAli/powerline,DoctorJellyface/powerline,s0undt3ch/powerline,russellb/powerline,junix/powerline,prvnkumar/powerline,xfumihiro/powerline,Liangjianghao/powerline,Luffin/powerline,S0lll0s/powerline,dragon788/powerline,blindFS/powerline,areteix/powerline,junix/powerline,QuLogic/powerline