Revert to `_or_empty` API in RemoveUnusedArgs
Revert to `_or_empty` API in RemoveUnusedArgs

Summary: There are cases that make this necessary.

Reviewed By: thezhangwei

Differential Revision: D31977677

fbshipit-source-id: fa6e47e597f8498b92c3b16748eef206e62e4589

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex