When an enum case has no members, there is no "contents" key, only "tag"

When an enum case has no members (nullary), there is no "contents" key, only "tag". Is that a problem for Argonaut? Does it always need the [`contents`](https://github.com/eskimor/purescript-argonaut-generic-codecs/blob/v6.0.4/src/Data/Argonaut/Generic/Aeson.purs#L48) key?