Fix system agent toggling in MySQL strict mode

Summary: `''` is not a valid integer.

Test Plan: Used `bin/accountadmin` to turn bot flag on and off for a user.

Reviewers: btrahan, Firehed

Reviewed By: Firehed

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9046

src/applications/people/editor/PhabricatorUserEditor.php

@@ -243,7 +243,7 @@ final class PhabricatorUserEditor extends PhabricatorEditor {
         $log->setOldValue($user->getIsSystemAgent());
         $log->setNewValue($system_agent);
 
-        $user->setIsSystemAgent($system_agent);
+        $user->setIsSystemAgent((int)$system_agent);
         $user->save();
 
         $log->save();