Allow to instrument blocks w/o source block
Allow to instrument blocks w/o source block

Reviewed By: agampe

Differential Revision: D26970282

fbshipit-source-id: 15dc8e2782fcaf2dcce49e144d5f1f199007d6c8

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