Avoid fatal if a macro has no file

Summary:
Not sure how this triggers, but we have a macro that has no file
associated with it, and this caused a fatal on the /macro endpoint.

Test Plan: https://phabricator.fb.com/macro/?page=100

Reviewers: vrana, nh, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D3642