[nullsafe][easy] use `lazy` instead of `fetcher ()`
[nullsafe][easy] use `lazy` instead of `fetcher ()`

Summary: This is more idiomatic in OCaml and hopefully a bit easier to read. Internally `lazy` will do pretty much the same thing but it also ensures the callback is called at most once.

Reviewed By: artempyanykh

Differential Revision: D21722522

fbshipit-source-id: 00897aaf1

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