rpc: fix RPC help requirements for getblocktemplate

First argument is optional, and defaults to {mode:template}.

Author: kallewoof

src/rpc/mining.cpp

@@ -302,7 +302,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
                 "    https://github.com/bitcoin/bips/blob/master/bip-0009.mediawiki#getblocktemplate_changes\n"
                 "    https://github.com/bitcoin/bips/blob/master/bip-0145.mediawiki\n",
                 {
-                    {"template_request", RPCArg::Type::OBJ, RPCArg::Optional::NO, "A json object in the following spec",
+                    {"template_request", RPCArg::Type::OBJ, "{}", "A json object in the following spec",
                         {
                             {"mode", RPCArg::Type::STR, /* treat as named arg */ RPCArg::Optional::OMITTED_NAMED_ARG, "This must be set to \"template\", \"proposal\" (see BIP 23), or omitted"},
                             {"capabilities", RPCArg::Type::ARR, /* treat as named arg */ RPCArg::Optional::OMITTED_NAMED_ARG, "A list of strings",