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