Clone as empty buffer when rocket::Payload holds null buffer pointer.
Clone as empty buffer when rocket::Payload holds null buffer pointer.

Reviewed By: zhxchen17

Differential Revision: D26290525

fbshipit-source-id: 1b2c03383c0b63cf72e8c5d0666bda2b0745eca4

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