R1:281bdf87cf63

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