fix: refactor TipsetKey using NonEmpty collection

[hanabi1224]

Summary of changes

Part of https://github.com/ChainSafe/forest/issues/3861 and a follow-up of https://github.com/ChainSafe/forest/pull/3934

Changes introduced in this pull request:

• refactor TipsetKey using NonEmpty collection

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• [x] I have performed a self-review of my own code,
• [x] I have made corresponding changes to the documentation,
• [x] I have added tests that prove my fix is effective or that my feature works (if possible),
• [x] I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[hanabi1224]

I'm a bit confused. I thought TipsetKey could be empty. Can you verify that the genesis block has a non-empty parent tipset key?

@lemmih As discussed, this is guaranteed on the implementation side, we might want to propose a small update to the spec later. (https://filecoinproject.slack.com/archives/C05P37R9KQD/p1709050261991189?thread_ts=1708351510.708049&cid=C05P37R9KQD)