/node/genesis_time return codes

[rolfyone]

Had a bit of a discussion around the failure state of having no genesis time to report on. It seems like this is more of an expected result than a failure result - pre-genesis the value won't be set.

Returning a 204 (no content) instead would be more appropriate potentially, and allows the API to still be queried without generating errors in this case.

In Teku we're following that theory currently, just because it seems to make sense.