Add nil checks to Versioned*BeaconBlock functions

[jtraglia]

This PR makes these changes:

• Add a lot of missing nil checks in the Versioned*BeaconBlock files.
• Change "no deneb block contents" to "no deneb block" which I think are copy/paste mistakes.
• Fix a couple "denb" typos.
• Handle DataVersionCapella case in BlobKZGCommitments function.
• Remove v.Capella.Message.Body.ExecutionPayload check in Grafitti function.

There are two competing style for these nil checks. I used whichever was more prevalent in the file. Personally I like the split conditionals better, but it doesn't really matter. If you'd like me to use the same style, I'd be happy to make the change.

I'm made this PR without reaching out first because I'm under the assumption these will never actually be nil.

[mcdee]

Looks good, thanks!