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

Affects the following RPCs:
- getblockheader
- getblock

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

src/rpc/blockchain.cpp

@@ -821,8 +821,8 @@ static RPCHelpMan getblockheader()
                             {RPCResult::Type::NUM, "difficulty", "The difficulty"},
                             {RPCResult::Type::STR_HEX, "chainwork", "Expected number of hashes required to produce the current chain"},
                             {RPCResult::Type::NUM, "nTx", "The number of transactions in the block"},
-                            {RPCResult::Type::STR_HEX, "previousblockhash", "The hash of the previous block"},
-                            {RPCResult::Type::STR_HEX, "nextblockhash", "The hash of the next block"},
+                            {RPCResult::Type::STR_HEX, "previousblockhash", /* optional */ true, "The hash of the previous block (if available)"},
+                            {RPCResult::Type::STR_HEX, "nextblockhash", /* optional */ true, "The hash of the next block (if available)"},
                         }},
                     RPCResult{"for verbose=false",
                         RPCResult::Type::STR_HEX, "", "A string that is serialized, hex-encoded data for block 'hash'"},
@@ -929,8 +929,8 @@ static RPCHelpMan getblock()
                     {RPCResult::Type::NUM, "difficulty", "The difficulty"},
                     {RPCResult::Type::STR_HEX, "chainwork", "Expected number of hashes required to produce the chain up to this block (in hex)"},
                     {RPCResult::Type::NUM, "nTx", "The number of transactions in the block"},
-                    {RPCResult::Type::STR_HEX, "previousblockhash", "The hash of the previous block"},
-                    {RPCResult::Type::STR_HEX, "nextblockhash", "The hash of the next block"},
+                    {RPCResult::Type::STR_HEX, "previousblockhash", /* optional */ true, "The hash of the previous block (if available)"},
+                    {RPCResult::Type::STR_HEX, "nextblockhash", /* optional */ true, "The hash of the next block (if available)"},
                 }},
                     RPCResult{"for verbosity = 2",
                 RPCResult::Type::OBJ, "", "",

test/functional/rpc_blockchain.py

@@ -304,6 +304,9 @@ class BlockchainTest(BitcoinTestFramework):
         header.calc_sha256()
         assert_equal(header.hash, besthash)
 
+        assert 'previousblockhash' not in node.getblockheader(node.getblockhash(0))
+        assert 'nextblockhash' not in node.getblockheader(node.getbestblockhash())
+
     def _test_getdifficulty(self):
         difficulty = self.nodes[0].getdifficulty()
         # 1 hash in 2 should be valid, so difficulty should be 1/2**31
@@ -408,6 +411,9 @@ class BlockchainTest(BitcoinTestFramework):
         # 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()