There is a very small number of release groups which have a first release date set, but have either no releases, or have none with dates, e.g.,

https://musicbrainz.org/release-group/10cc8735-f9b7-3619-9181-41868c9feae9

https://musicbrainz.org/release-group/1025abf7-8405-47c3-ac02-508b078207b7

(You have to view the WS response to see the first-release-date.) We should recalculate those columns in release_group_meta.

Note that I didn't find any RGs where the first release date is blank but shouldn't be.