### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

The help text says:

```
  replaceable(=N)
       Set RBF opt-in sequence number for input N (if not provided, opt-in all
       available inputs)
```

but when I don't provide N, I get an error:

```
$ bitcoin-tx -create in=0000000000000000000000000000000000000000000000000000000000000000:0 replaceable
error: Invalid TX input index ''
```

### Expected behaviour

I expect bitcoin-tx to act like its help string describes:

```
$ bitcoin-tx -create in=0000000000000000000000000000000000000000000000000000000000000000:0 replaceable
020000000100000000000000000000000000000000000000000000000000000000000000000000000000fdffffff0000000000
```

The fix is simple:

```
diff --git a/src/bitcoin-tx.cpp b/src/bitcoin-tx.cpp
index a05443a2e5..b9348fcad0 100644
--- a/src/bitcoin-tx.cpp
+++ b/src/bitcoin-tx.cpp
@@ -222,7 +222,7 @@ static void MutateTxRBFOptIn(CMutableTransaction& tx, const std::string& strInId
 {
     // parse requested index
     int64_t inIdx;
-    if (!ParseInt64(strInIdx, &inIdx) || inIdx < 0 || inIdx >= static_cast<int64_t>(tx.vin.size())) {
+    if (strInIdx != "" && (!ParseInt64(strInIdx, &inIdx) || inIdx < 0 || inIdx >= static_cast<int64_t>(tx.vin.size()))) {
         throw std::runtime_error("Invalid TX input index '" + strInIdx + "'");
     }
 
```

### Steps to reproduce

Run the command shown above.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

master@e3eb3aae3

### Operating system and version

Debian 12

### Machine specifications

_No response_
const int nOut = prevOut["vout"].getInt<int>();
            if (nOut < 0)
                throw std::runtime_error("vout cannot be negative");
            COutPoint out(Txid::FromUint256(txid), nOut);
            std::vector<unsigned char> pkData(ParseHexUV(prevOut["scriptPubKey"], "scriptPubKey"));
            CScript scriptPubKey(pkData.begin(), pkData.end());
            {
                const Coin& coin = view.AccessCoin(out);
                if (!coin.IsSpent() && coin.out.scriptPubKey != scriptPubKey) {
                    std::string err("Previous output scriptPubKey mismatch:\n");
                    err = err + ScriptToAsmStr(coin.out.scriptPubKey) + "\nvs:\n"+
                        ScriptToAsmStr(scriptPubKey);
                    throw std::runtime_error(err);
                }
                Coin newcoin;
                newcoin.out.scriptPubKey = scriptPubKey;
                newcoin.out.nValue = MAX_MONEY;
                if (prevOut.exists("amount")) {
                    newcoin.out.nValue = AmountFromValue(prevOut["amount"]);
                }
                newcoin.nHeight = 1;
                view.AddCoin(out, std::move(newcoin), true);
            }
            
            
            if ((scriptPubKey.IsPayToScriptHash() || scriptPubKey.IsPayToWitnessScriptHash()) &&
                prevOut.exists("redeemScript")) {
                UniValue v = prevOut["redeemScript"];
                std::vector<unsigned char> rsData(ParseHexUV(v, "redeemScript"));
                CScript redeemScript(rsData.begin(), rsData.end());