The ellipsis operator was used as a hack to not need to pass any details
as an argument, but it makes what the end object will actually look like
less obvious. It also makes it impossible to pass an array to details
without getting a nested array.
Fixes#2874
Ensure that `UnmarshalTargetResultOptions()` is called only when there
are any options to unmarshal in the JSON object.
Since results of some Targets don't have any options defined, mark
`TargetResult.Options` as optional in the JSON tag.