### 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_
UniValue keysObj = registers["privatekeys"];
    for (unsigned int kidx = 0; kidx < keysObj.size(); kidx++) {
        if (!keysObj[kidx].isStr())
            throw std::runtime_error("privatekey not a std::string");
        CKey key = DecodeSecret(keysObj[kidx].getValStr());
        if (!key.IsValid()) {
            throw std::runtime_error("privatekey not valid");
        }
        tempKeystore.AddKey(key);
    }
    
    if (!registers.count("prevtxs"))
        throw std::runtime_error("prevtxs register variable must be set.");
    UniValue prevtxsObj = registers["prevtxs"];
    {
        for (unsigned int previdx = 0; previdx < prevtxsObj.size(); previdx++) {
            const UniValue& prevOut = prevtxsObj[previdx];
            if (!prevOut.isObject())
                throw std::runtime_error("expected prevtxs internal object");
            std::map<std::string, UniValue::VType> types = {
                {"txid", UniValue::VSTR},
                {"vout", UniValue::VNUM},
                {"scriptPubKey", UniValue::VSTR},
            };
            if (!prevOut.checkObject(types))
                throw std::runtime_error("prevtxs internal object typecheck fail");
            uint256 txid;
            if (!ParseHashStr(prevOut["txid"].get_str(), txid)) {
                throw std::runtime_error("txid must be hexadecimal string (not '" + prevOut["txid"].get_str() + "')");
            }