Change log date for unpublished releases

The changelog command sets the date to 01-01-1970 for unpublished releases

https://github.com/github-tools/github-release-notes/blob/ff0ddae3938db3cfcca5ef317d74eee034c32cc5/lib/src/Gren.js#L499

Maybe it should fallback to a different date if `release.published_at` is `null`

Start at lib/src/Gren.js line 499, where the changelog command handles the release date. Inspect how unpublished releases with a null release.published_at are formatted, then determine the appropriate fallback date and verify that the generated changelog no longer uses 01-01-1970 for those releases.

索引モデルが issue の本文から書いたものです。