[fix bug 1129162] Respect blank value of model field.
[fix bug 1129162] Respect blank value of model field.

liu21st/masterfirefoxos,mozilla/masterfirefoxos,craigcook/masterfirefoxos,enng0227/masterfirefoxos,craigcook/masterfirefoxos,mozilla/masterfirefoxos,mozilla/masterfirefoxos,glogiotatidis/masterfirefoxos,glogiotatidis/masterfirefoxos,glogiotatidis/masterfirefoxos,liu21st/masterfirefoxos,enng0227/masterfirefoxos,craigcook/masterfirefoxos,mozilla/masterfirefoxos,liu21st/masterfirefoxos,enng0227/masterfirefoxos,craigcook/masterfirefoxos,enng0227/masterfirefoxos,glogiotatidis/masterfirefoxos,liu21st/masterfirefoxos