[sledge] Add fast path to Context.Subst.compose for empty
[sledge] Add fast path to Context.Subst.compose for empty

Reviewed By: jvillard

Differential Revision: D25756568

fbshipit-source-id: 7f1856279

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