Fix #364. In WSH, return empty string for files not found.
Fix #364. In WSH, return empty string for files not found.

kmulvey/bootstrap-csslint,sergeychernyshev/csslint,dmi3y/csslint,scott1028/csslint,malept/csslint,SimenB/csslint,krahman/csslint,sergeychernyshev/csslint,malept/csslint,dashk/csslint,Arcanemagus/csslint,YanickNorman/csslint,scott1028/csslint,lovemeblender/csslint,YanickNorman/csslint,dashk/csslint,SimenB/csslint,lovemeblender/csslint,codeclimate/csslint,Arcanemagus/csslint,codeclimate/csslint