PrettyPatch.rb complains about missing checksum for new pixel results

PrettyPatch.rb complains about missing checksum for new pixel results
https://bugs.webkit.org/show_bug.cgi?id=88368

Reviewed by Tony Chang.

When adding or removing a file, we incorrectly iterpreted not having an image
as not having a checksum.

* PrettyPatch/PrettyPatch.rb:
* PrettyPatch/PrettyPatch_test.rb:
I tried to fix the TempFile issue in these tests, but after a couple hours
of banging my head against this, I have no idea what's breaking.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@122607 268f45cc-cd09-0410-ab3c-d52691b4dbfc