Merge bitcoin#21053: rpc, test: document {previous,next}blockhash as optional

ba7e17e 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").

Top commit has no ACKs.

Tree-SHA512: ef42c5c773fc436e1b4a67be14e2532e800e1e30e45e54a57431c6abb714d2c069c70d40ea4012d549293b823a1973b3f569484b3273679683b28ed40abf46bb

Author: MarcoFalke

test/functional/rpc_blockchain.py

@@ -444,6 +444,9 @@ def move_block_file(old, new):
         # Restore chain state
         move_block_file('rev_wrong', 'rev00000.dat')
 
+        assert 'previousblockhash' not in node.getblock(node.getblockhash(0))
+        assert 'nextblockhash' not in node.getblock(node.getbestblockhash())
+
 
 if __name__ == '__main__':
     BlockchainTest().main()