  "mempoolminfee": 0.00000000

When the minrelaytxfee is > 0.

{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            "getblockhash height\n"
            "\nReturns hash of block in best-block-chain at height provided.\n"
            "\nArguments:\n"
            "1. height         (numeric, required) The height index\n"
            "\nResult:\n"
            "\"hash\"         (string) The block hash\n"
            "\nExamples:\n"
            + HelpExampleCli("getblockhash", "1000")
            + HelpExampleRpc("getblockhash", "1000")
        );
    LOCK(cs_main);
    int nHeight = request.params[0].get_int();
    if (nHeight < 0 || nHeight > chainActive.Height())
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Block height out of range");
    CBlockIndex* pblockindex = chainActive[nHeight];
    return pblockindex->GetBlockHash().GetHex();
}
UniValue getblockheader(const JSONRPCRequest& request)