getHeader: 204 (no content) response if no header is available

[metachris]

See also #32

If no header is available, status code 204 and no content can be returned. 204 seems appropriate: https://www.webfx.com/web-development/glossary/http-status-codes/what-is-a-204-status-code/

Would be returned from the relay if it has no header naturally, or during the period after the merge where we manually disable headers.

[ralexstokes]

I think 204 fits for this use case

although I would also think it is fine to just return an error, rather than add more feature surface into the builder specs

wdyt?

edit: thought about this a bit more and you could easily have a relay w/o a header (e.g. builders didn't supply one for some reason) for a given slot and then I think a 204 would make sense here

[metachris]

Also if the relay is not shipping blocks for a few epochs after the merge, I'd rather not return an error on every block (which will pollute the mev-boost and BN logs with errors as well). A clean no-content response would be much preferable imo.

[lightclient]

This makes a lot of sense to me.