Add comment to CSV output, allow fields to be optional

Some checks have a comment attribute that adds helpful context to findings: https://gitlab.com/gitlab-com/cs-tools/gitlab-cs-tools/deprecation-migration-tools/advanced-search-deprecations/-/blob/main/18.0_config.yml?ref_type=heads#L48-55 – but currently we haven't added this to the output, so it wasn't really discoverable by users.

Because comment is optional, I noticed that we should change from e.g. deprecation_data["name"], to deprecation_data.get("name", ''), – otherwise a finding simply wouldn't be included in the output CSV if the field isn't present in a check. (It'd show a Error writing finding to CSV: 'comment' instead, but that could be easy to miss.) While name, doc and issue should hopefully always be present, it's still better to never drop a finding just because a check lacks full metadata.