Fix Herald field type error

Summary: See IRC. This is a likely fix for @DctrWatson's error: we're returning `null` but should return `array()` to indicate no values.

Test Plan: Will make @DctrWatson do it.

Reviewers: btrahan

Reviewed By: btrahan

CC: dctrwatson, aran

Differential Revision: https://secure.phabricator.com/D7950