api/types/image: make InspectResponse.GraphDriver optional

[austinvazquez]

- What I did
Split from #50857

This change makes the GraphDriver field in image.InspectResponse optional. This field will only be returned when using moby engine graph drivers as a backend storage implementation. It will be omitted when using the containerd image backend.

- How I did it

- How to verify it

- Human readable description for the release notes

api: make `GraphDriver` field in `image.InspectResponse` optional. This field will continue to be emitted when using the legacy graph drivers and will be omitted when using the containerd image backend.

- A picture of a cute animal (not mandatory but encouraged)

[dmcgowan]

Omit empty is respected on non-pointer structs if all the sub-fields of the struct are also empty?

[austinvazquez]

Good catch I think you are right it would be not be effective here.

[thaJeztah]

Needs a rebase, @austinvazquez

[thaJeztah]

docker-py test failures should also be checked probably; they could be legit (and possibly have to be adjusted, and temporarily skipped)

cc @ctalledo

=================================== FAILURES ===================================
_________________ ServiceTest.test_create_service_with_config __________________
tests/integration/api_service_test.py:767: in test_create_service_with_config
    assert container is not None
E   assert None is not None
_________________ ServiceTest.test_create_service_with_secret __________________
tests/integration/api_service_test.py:715: in test_create_service_with_secret
    assert container is not None
E   assert None is not None

[austinvazquez]

@thaJeztah , looks like it was a flake. It reran on the rebase.

[thaJeztah]

LGTM