### 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_
ECC_Start();
    }
    ~Secp256k1Init() {
        ECC_Stop();
    }
};
static void MutateTx(CMutableTransaction& tx, const std::string& command,
                     const std::string& commandVal)
{
    std::unique_ptr<Secp256k1Init> ecc;
    if (command == "nversion")
        MutateTxVersion(tx, commandVal);
    else if (command == "locktime")
        MutateTxLocktime(tx, commandVal);
    else if (command == "replaceable") {
        MutateTxRBFOptIn(tx, commandVal);
    }
    else if (command == "delin")
        MutateTxDelInput(tx, commandVal);
    else if (command == "in")
        MutateTxAddInput(tx, commandVal);
    else if (command == "delout")
        MutateTxDelOutput(tx, commandVal);
    else if (command == "outaddr")
        MutateTxAddOutAddr(tx, commandVal);
    else if (command == "outpubkey") {
        ecc.reset(new Secp256k1Init());
        MutateTxAddOutPubKey(tx, commandVal);
    } else if (command == "outmultisig") {