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

src/applications/maniphest/command/ManiphestAssignEmailCommand.php

@@ -34,6 +34,7 @@ final class ManiphestAssignEmailCommand
     array $argv) {
     $xactions = array();
 
+    $assign_phid = null;
 
     $assign_to = head($argv);
     if ($assign_to) {