Fix "Welcome Mail" check for a message when no message exists

Summary: Fixes T13239. See that task for discussion.

Test Plan: Tried to send welcome mail with no "Welcome" message.

Maniphest Tasks: T13239

Differential Revision: https://secure.phabricator.com/D20001

src/applications/people/controller/PhabricatorPeopleWelcomeController.php

@@ -51,7 +51,7 @@ final class PhabricatorPeopleWelcomeController
     $default_message = PhabricatorAuthMessage::loadMessage(
       $admin,
       PhabricatorAuthWelcomeMailMessageType::MESSAGEKEY);
-    if (strlen($default_message->getMessageText())) {
+    if ($default_message && strlen($default_message->getMessageText())) {
       $message_instructions = pht(
         'The email will identify you as the sender. You may optionally '.
         'replace the [[ %s | default custom mail body ]] with different text '.