### 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_
}
    if (scriptPubKey.size() > MAX_SCRIPT_SIZE) {
        throw std::runtime_error(strprintf(
                    "script exceeds size limit: %d > %d", scriptPubKey.size(), MAX_SCRIPT_SIZE));
    }
    if (bSegWit) {
        scriptPubKey = GetScriptForDestination(WitnessV0ScriptHash(scriptPubKey));
    }
    if (bScriptHash) {
        if (scriptPubKey.size() > MAX_SCRIPT_ELEMENT_SIZE) {
            throw std::runtime_error(strprintf(
                        "redeemScript exceeds size limit: %d > %d", scriptPubKey.size(), MAX_SCRIPT_ELEMENT_SIZE));
        }
        scriptPubKey = GetScriptForDestination(ScriptHash(scriptPubKey));
    }
    
    CTxOut txout(value, scriptPubKey);
    tx.vout.push_back(txout);
}
static void MutateTxDelInput(CMutableTransaction& tx, const std::string& strInIdx)
{
    
    int64_t inIdx;
    if (!ParseInt64(strInIdx, &inIdx) || inIdx < 0 || inIdx >= static_cast<int64_t>(tx.vin.size())) {
        throw std::runtime_error("Invalid TX input index '" + strInIdx + "'");
    }
    
    tx.vin.erase(tx.vin.begin() + inIdx);
}
static void MutateTxDelOutput(CMutableTransaction& tx, const std::string& strOutIdx)