Fix a fatal for 404s in Phriction

Summary:
Fixes T7817. Not all documents have `$content`, so don't try to set a timestamp if there's no content.

An example is `/w/asdlkfnalskdnfaslk/`, i.e. the 404 document.

Test Plan: Hit `/w/junk/` and got a nice 404 instead of a fatal.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7817

Differential Revision: https://secure.phabricator.com/D12401

src/applications/phriction/controller/PhrictionDocumentController.php

@@ -35,6 +35,7 @@ final class PhrictionDocumentController
     $core_content = '';
     $move_notice = '';
     $properties = null;
+    $content = null;
 
     if (!$document) {
 
@@ -194,8 +195,11 @@ final class PhrictionDocumentController
     $header = id(new PHUIHeaderView())
       ->setUser($user)
       ->setPolicyObject($document)
-      ->setHeader($page_title)
+      ->setHeader($page_title);
-      ->setEpoch($content->getDateCreated());
+
+    if ($content) {
+      $header->setEpoch($content->getDateCreated());
+    }
 
     $prop_list = null;
     if ($properties) {