NPE with Docker manifests that contain null for a "Cmd" field

Some docker builds may produce an image that will make the docker client produce a manifest.json that has some null values in it. Specifically the "Cmd" field is important to us, and a null for a value for this field will cause a NPE when rendering the "Docker Info" tab on the UI, which means it won't render in the UI.

We should fix this NPE; if such a manifest is pushed to Artifactory, we can print something like "null" or "manifest doesn't contain Cmd info" for the CMD row in the Docker Info tab.