AsyncProcessor now returns an empty buffer instead of null when returning "void"
AsyncProcessor now returns an empty buffer instead of null when returning "void"

Summary:
Messed up a diff merge and left this change out of D21914313

(Note: this ignores all push blocking failures!)

Reviewed By: stevegury, avalonalex

Differential Revision: D22464460

fbshipit-source-id: 1925a15984465a8bb68439fda89cd400b65a69ca

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift