When compiling without external signer support, the resulted `bench_bitcoin.exe` fails on Windows:
```
>.\bench_bitcoin.exe -filter=AddrManSelectByNetwork
Error: bad optional access
```

Steps to reproduce the failure:
- cross compiling on Ubuntu 22.04:
```
$ make -j $(nproc) -C depends HOST=x86_64-w64-mingw32 NO_QT=1
$ ./autogen.sh
$ ./configure CONFIG_SITE=$PWD/depends/x86_64-w64-mingw32/share/config.site --disable-external-signer
$ make -j $(nproc)
```
- compiling natively on Windows:

Apply the diff as follows:
```diff
--- a/build_msvc/bitcoin_config.h.in
+++ b/build_msvc/bitcoin_config.h.in
@@ -41,9 +41,6 @@
 /* Define this symbol to enable ZMQ functions */
 #define ENABLE_ZMQ 1
 
-/* define if external signer support is enabled (requires Boost::Process) */
-#define ENABLE_EXTERNAL_SIGNER /**/
-
 /* Define to 1 if you have the declaration of `be16toh', and to 0 if you
    don't. */
 #define HAVE_DECL_BE16TOH 0
```
Then
```
>py -3 build_msvc\msvc-autogen.py
>msbuild build_msvc\bitcoin.sln -property:Configuration=Release -verbosity:minimal
>src\bench_bitcoin.exe -filter=AddrManSelectByNetwork
Error: bad optional access
```

---

The failure does not manifest itself when running `bench_bitcoin.exe` using Wine on Linux.
const std::vector<COutPoint>& inputs,
                                                                                  int input_height,
                                                                                  const std::vector<CKey>& input_signing_keys,
                                                                                  const std::vector<CTxOut>& outputs,
                                                                                  const std::optional<CFeeRate>& feerate,
                                                                                  const std::optional<uint32_t>& fee_output)
{
    CMutableTransaction mempool_txn;
    mempool_txn.vin.reserve(inputs.size());
    mempool_txn.vout.reserve(outputs.size());
    for (const auto& outpoint : inputs) {
        mempool_txn.vin.emplace_back(outpoint, CScript(), MAX_BIP125_RBF_SEQUENCE);
    }
    mempool_txn.vout = outputs;
    
    FillableSigningProvider keystore;
    for (const auto& input_signing_key : input_signing_keys) {
        keystore.AddKey(input_signing_key);