fix(rad): Fix crash when a param doesn't specify a type

[dazuma]

Fixes googleapis/doc-pipeline#423

The issue, apparently, is that when a @param tag is not annotated with a type, the types field is set to nil, which causes a crash here because we're calling map on the result. Generally, we always include a type for a @param tag, even if it's just [Object], but it looks like one slipped through in https://github.com/googleapis/google-cloud-ruby/pull/21002/files (dataset.rb line 990). I'll fix that separately, but for docgen, we can make it robust against that case by calling to_a on the types to map nil to the empty array before calling map on it.

Also updates the release script so failures in build-rad cause a full abort of publish-rad rather than letting it attempt to publish an incomplete tarball.