When a diff contained a newly-added empty file, the resulting diff

entry would include a download link, which then crashed with a

PatchError. We've been getting these error emails a lot as bots have

crawled our server.
This change applies two fixes:

We no longer show a download link for new empty files.
If somehow the download endpoint is accessed anyway, we catch

PatchError and return a 404.

Verified that newly-added empty files no longer contained a download

  link.
Ran unit tests.