Initialize `$assign_phid` properly in the "!assign" email action
Summary: If you `!assign cahd` when you meant to `!assign chad`, we'll hit an "Undefined variable: assign_phid" a little further down.
Test Plan: Eyeballed it. See IRC.
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D14291