rpc, test: document {previous,next}blockhash as optional

Summary:
Affects the following RPCs:

• getblockheader
• getblock

Also adds trivial tests on genesis block (should not contain "previousblockhash") and best block (should not contain "nextblockhash").

This is a backport of core#21053

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11336