Merge bitcoin/bitcoin#26955: wallet: permit mintxfee=0

f11eb1f wallet: permit mintxfee=0 (willcl-ark)

Pull request description:

  Fixes #26797

  Permit nodes to use `-mintxfee=0`. Values below 0 are handled by the ParseMoney() check.

ACKs for top commit:
  MarcoFalke:
    review ACK f11eb1f
  john-moffett:
    ACK f11eb1f

Tree-SHA512: 3bf50362bced4fee8e3a846cfb46f1c65dd607c9c824aa3f8c52294371b0646d167a04772d5302bdbee35bbaf407ef0aa634228f70e522c3e423f4213b4ae071

src/wallet/wallet.cpp

@@ -2995,7 +2995,7 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
 
     if (args.IsArgSet("-mintxfee")) {
         std::optional<CAmount> min_tx_fee = ParseMoney(args.GetArg("-mintxfee", ""));
-        if (!min_tx_fee || min_tx_fee.value() == 0) {
+        if (!min_tx_fee) {
             error = AmountErrMsg("mintxfee", args.GetArg("-mintxfee", ""));
             return nullptr;
         } else if (min_tx_fee.value() > HIGH_TX_FEE_PER_KB) {