fix that css binding does not update the GUI when observable set to null
fix that css binding does not update the GUI when observable set to null

skyh/knockout,moonpyk/knockout,skyh/knockout,stonegithubs/knockout,getshuvo/knockout,youprofit/knockout,ErikSchierboom/knockout,gwokudasam/knockout,gwokudasam/knockout,hitesh97/knockout,youprofit/knockout,getshuvo/knockout,martijnwetering/knockout,DekeYan/knockout,xeronith/knockout,cgourlay/knockout,stonegithubs/knockout,bago/knockout,JesseQin/knockout,brianmhunt/knockout,hitesh97/knockout,ErikSchierboom/knockout,moonpyk/knockout,DekeYan/knockout,bago/knockout,cgourlay/knockout,mattbalmer/knuckles,JesseQin/knockout,martijnwetering/knockout,mattbalmer/knuckles,Sir0xb/knockout,pianove/knockout,pianove/knockout,xeronith/knockout,Sir0xb/knockout,brianmhunt/knockout