Allow ddljson response to not contain an image

Interactive doodles don't use the image (data_uri field) included
in the ddljson response. Don't require that responses (on Desktop)
contain an image. Allow logo_cache to cache a doodle that doesn't
include an image.

Bug: 800170
Change-Id: I637ab2130db29efadecebd787c244e59008d3e63
Reviewed-on: https://chromium-review.googlesource.com/c/1405431
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623783}
9 files changed