Minor, don't show references in diffusion if there isn't any

Summary:
When there are no other references (and there will be none ususally,
if D2891 is accepted), we would show "References:" with empty contents.

Avoid that by testing for empty output after applying stupid filtering.

Test Plan: Looked in a standard repository with no special refs.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2892