[sledge] Add fast path for applying an empty substitution
[sledge] Add fast path for applying an empty substitution

Reviewed By: jvillard

Differential Revision: D25756576

fbshipit-source-id: 401eab0c0

jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer