R1:99d972fc8187

Fix Herald rule actions on empty custom PHID fields

Summary: Fixes T9260. That task has a good description of the issue.

Test Plan: Followed steps in T9260 to reproduce the issue. Applied patch; issue went away.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9260

Differential Revision: https://secure.phabricator.com/D14169