Fix pulling images that contain no layers at all

The download manager assumed there was at least one layer involved in
all images. This can be false if the image is essentially a copy of
`scratch`.

Fix a nil pointer dereference that happened in this case. Add
integration tests that involve schema1 and schema2 manifests.

Fixes #21213

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>