Merge bitcoin/bitcoin#23547: Bugfix: RPC/mining: Fail properly in estimatesmartfee if smart fee data is unavailable

cd8d156 Bugfix: RPC/mining: Fail properly in estimatesmartfee if smart fee data is unavailable (Luke Dashjr)

Pull request description:

  Fixes a regression introduced by #22722

  (Not entirely sure on the solution)

ACKs for top commit:
  prayank23:
    crACK bitcoin/bitcoin@cd8d156
  darosior:
    utACK cd8d156
  kristapsk:
    utACK cd8d156

Tree-SHA512: eb4aa3cc345c69c44ffd5733b51b90eefe1d7854b7a2855e8cbb98268db24d43b7d0ae9fbb0eccf9b6dc01da644d19433cc77fec52ff67bf890be1fc53a67fc4

Author: MarcoFalke

src/rpc/mining.cpp

@@ -1121,10 +1121,10 @@ static RPCHelpMan estimatesmartfee()
     UniValue errors(UniValue::VARR);
     FeeCalculation feeCalc;
     CFeeRate feeRate{fee_estimator.estimateSmartFee(conf_target, &feeCalc, conservative)};
-    CFeeRate min_mempool_feerate{mempool.GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000)};
-    CFeeRate min_relay_feerate{::minRelayTxFee};
-    feeRate = std::max({feeRate, min_mempool_feerate, min_relay_feerate});
     if (feeRate != CFeeRate(0)) {
+        CFeeRate min_mempool_feerate{mempool.GetMinFee(gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000)};
+        CFeeRate min_relay_feerate{::minRelayTxFee};
+        feeRate = std::max({feeRate, min_mempool_feerate, min_relay_feerate});
         result.pushKV("feerate", ValueFromAmount(feeRate.GetFeePerK()));
     } else {
         errors.push_back("Insufficient data or no feerate found");