Return None when principal user is not available rather than causing a confusing attribute error
Return None when principal user is not available rather than causing a confusing attribute error

Summary: [Thrift] Return None when principal user is not available rather than causing a confusing attribute error

Reviewed By: yfeldblum

Differential Revision: D3327518

fbshipit-source-id: 65ce7e84baac70c462de0537785f5a00fd6c601b

LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,facebook/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,facebook/fbthrift,facebook/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift