message: fix rpcmining/getblocktemplate univalue transition logic error
  This has little to no impact on PIVX since we don't use
  `getblocktemplate` at all and are not PoW, but including it here as a
  consistency update from upstream.
diff: @@ -382,7 +382,7 @@ UniValue getblocktemplate(const UniValue& params, bool fHelp)
  if (strMode == "proposal") {
  const UniValue& dataval = find_value(oparam, "data");
  - if (dataval.isStr())
  + if (!dataval.isStr())
  throw JSONRPCError(RPC_TYPE_ERROR, "Missing data String key for proposal");
  CBlock block;
  