Only use observerLossEvent if there are observers.
Only use observerLossEvent if there are observers.

Summary: As in title. This makes it a noop if the connection has no observers.

Reviewed By: FelixMerk

Differential Revision: D32069244

fbshipit-source-id: a9bb6d22a2b432729c0bac586125f06d8a104b9e

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst