Construct default functions by passing nothing rather than 0
Construct default functions by passing nothing rather than 0

Summary:
Because MSVC's implementation of `std::function` is really not a big fan of passing in `0`, more specifically, it errors at the end of a 15 deep template chain. Constructing it by passing nothing solves the problem.
Closes https://github.com/facebook/fbthrift/pull/108

Reviewed By: juchem

Differential Revision: D2853355

Pulled By: Orvid

fb-gh-sync-id: 5e154cc6d45d3cf5312ba7ee144d289f61b30125

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