Add support for nullable return types

Summary: Ref T4334. Adds support for nullable return type, which are a thing as of PHP 7.1.

Test Plan: Updated test cases.

Reviewers: epriestley, richardvanvelzen, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: thaiphv, wjiang, Korvin, Firehed

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D18621