Given: You have an ImageStream that has no docker image repository (#setDockerImageRepository was never called & the json that the object was constructed from does neither contain the property "spec.dockerImageRepository" nor "status.dockerImageRepository")
When: call #getDockerImageRepository
Then: you get "latest"

This seems wrong to me. It should - I believe return null.
This looks like an issue in DockerImageURI to me which doesnt check if the repo url is null and blindly appends the tag ("latest").