Fixed tricky bug in edxTrackLogJSONParser:handleOpenAssessmentEvent in which resource_display_name was set to None, and then that None was tested with the len() func.
Fixed tricky bug in edxTrackLogJSONParser:handleOpenAssessmentEvent in which resource_display_name was set to None, and then that None was tested with the len() func.

paepcke/json_to_relation,paepcke/json_to_relation,paepcke/json_to_relation,paepcke/json_to_relation