containertool: Use a null object to represent scratch images

[euanh]

Motivation

When the --from scratch option is used, containertool does not download the scratch from Docker Hub - instead it synthesizes a suitable empty image. Currently this is handled by making the source client optional, but that causes nil checks to spread everywhere. Now that we have the ImageSource protocol (#148) we can instead use a null object which can only return an empty image. This allows the nil checks to be removed and lets the same code path handle images built on real base images and the scratch base image.

Modifications

• Add ScratchImage, a null object implementing the ImageSource protocol
• Remove special cases previously needed to make the image source optional.
• Remove the option to pass JSONEncoder and JSONDecoder instances to RegistryClient. Specific configuration options must be used to match the requirements of the image specification, so there is not much value in allowing the user to pass in an encoder with an unknown configuration.

Result

• Fewer special cases
• No functional change

Test Plan

Existing tests continue to pass.