ba7e17e073 rpc, test: document {previous,next}blockhash as optional (Sebastian Falbesoner)
Pull request description:
This PR updates the result help of the following RPCs w.r.t. the `previousblockhash` and `nextblockhash` fields:
- getblockheader
- getblock
Also adds trivial tests on genesis block (should not contain "previousblockhash") and best block (should not contain "nextblockhash").
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