### 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_
{
    std::string strHex = EncodeHexTx(tx);
    tfm::format(std::cout, "%s\n", strHex);
}
static void OutputTx(const CTransaction& tx)
{
    if (gArgs.GetBoolArg("-json", false))
        OutputTxJSON(tx);
    else if (gArgs.GetBoolArg("-txid", false))
        OutputTxHash(tx);
    else
        OutputTxHex(tx);
}
static std::string readStdin()
{
    char buf[4096];
    std::string ret;
    while (!feof(stdin)) {
        size_t bread = fread(buf, 1, sizeof(buf), stdin);
        ret.append(buf, bread);
        if (bread < sizeof(buf))
            break;
    }
    if (ferror(stdin))
        throw std::runtime_error("error reading stdin");
    return TrimString(ret);
}
static int CommandLineRawTx(int argc, char* argv[])