R1:c724902ca9da

Don't fail with no image macros

Summary: .. IN (%Ls) with no file phids fails miserably.

Test Plan:
Went to /file/macro/ with and without patch. An exception is thrown
without it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1063