fix parser value check bug.
fix parser value check bug.

value shall not be None but can be empty string

itnihao/thriftpy,adsharma/flattools,duydb2/thriftpy,importcjj/thriftpy,duydb2/thriftpy,dan-blanchard/thriftpy,mariusvniekerk/thriftpy,importcjj/thriftpy,eleme/thriftpy,spladug/thriftpy,halfcrazy/thriftpy,adsharma/flattools,mariusvniekerk/thriftpy,duydb2/thriftpy,eleme/thriftpy,misakwa/thriftpy,maralla/thriftpy,halfcrazy/thriftpy,maralla/thriftpy,itnihao/thriftpy,maralla/thriftpy,OctavianLee/thriftpy,misakwa/thriftpy,spladug/thriftpy,misakwa/thriftpy,keitheis/thriftpy,mariusvniekerk/thriftpy,OctavianLee/thriftpy,OctavianLee/thriftpy,keitheis/thriftpy,dan-blanchard/thriftpy,itnihao/thriftpy,adsharma/flattools