Add PHPAST test for empty heredoc

Summary: Ref T4334. Ref T3512. This result is wrong, but cover it with a test before I muck around with the parser itself.

Test Plan: Ran unit tests.

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3512, T4334

Differential Revision: https://secure.phabricator.com/D8029