Fix node for nullable return value

Summary: Ref T4334. Fixes a bug introduced by D18621 which created a node without a valid type.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, Firehed, wjiang

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D18639