SaveImagesCmd and optional vs. mandatory tag names #1872

The JavaDoc of SaveImagesCmd.TaggedImage describes that it could be just a name, or name and tag separated by a colon.
(the former would save all tags for the name to the tar file)

But SaveImagesCmd#withImage requires tag as @NonNull
and the SaveImagesCmdImpl.TaggedImageImpl also does not consider the described case without tag.

The SaveImageCmd supports the tag-less save without problem.

I think the @NonNull restriction for the tag should be lifted and the implementation adapted accordingly.