don't throw BadMethodCallException on diffs that add images

Summary:
I don't know if there is something more sinister going on
under the covers, but we have a couple of diffs that trigger:

Unhandled Exception ("BadMethodCallException")
Call to a member function getMetadata() on a non-object

when the diff page is handling its async render calls. One diff
in particular has multiple image adds and thus has a stack of of these
error dialogs to close.

This isn't a new regression, we just haven't gotten around to debugging
it until now (reported on 6/12)

One revision that triggers it has two diffs. If I show Base -> Diff 1
I don't hit the error. When I select Base -> Diff 2, or Diff 1 -> Diff
2, the error triggers.

I don't understand what this means, but this diff avoids the null object
reference that causes the exception.

Test Plan:
Load the offending diff, don't hit the error. The diff loads
the images that were added

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6851