Don't specify size 0 for deleted files

Summary: See D15828 - arc is reporting file size as 0 for unexisting files - make it stop.

Test Plan: arc diff with empty, deleted, added files - see size reported as null when appropriate.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: chad, Korvin

Differential Revision: https://secure.phabricator.com/D16059