`crane mutate` deletes Cmd if you try to override the `--entrypoint` without `--cmd`

### Describe the bug

Given a base image with Entrypoint + Cmd;

`crane mutate --entrypoint ...` without also specifying `--cmd` deletes the `Cmd`. You end up with an Entrypoint only.

### To Reproduce

As above

### Expected behavior

Expected it to preserve the original Cmd, and override the Entrypoint only.

Deleting Cmd is unexpected, Dockerfile syntax `ENTRYPOINT [...]` does not do that. Ultimately to use `crane mutate`, you have to look up the base image, get .Config.Cmd, and re-add it as --cmd if you are using --entrypoint alone. This is possible! But not expected behaviour

### Additional context

Add any other context about the problem here.

- Output of `crane version`: 0.19.1, but there are no relevant commits since then.

Start with the `crane mutate` command and reproduce the case using a base image with both Entrypoint and Cmd, providing only `--entrypoint`. Trace the handling of `--entrypoint` and `--cmd`; done means the original Cmd remains when only the Entrypoint is overridden, with regression coverage for this behavior.

Written by the indexing model from the issue text.