wallet: permit mintxfee=0

Fixes #26797

Permit nodes to use a mintxfee of `0` if they choose.
Values below 0 are handled by the ParseMoney() check.

Author: willcl-ark

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) {