Empty tuple conversion fails to compile

Automatic conversion between std::tuple<> and boost::json::value does not work.
The code

```
std::tuple<> t0;
boost::json::value v = boost::json::value_from(t0);
std::tuple<> t1 = boost::json::value_to>(v);
```

fails to compile with the error message

`static assertion failed: 'No suitable tag_invoke overload found for the type'`.

This seems counterintuitive. Obviously an empty C++ tuple should be converted to an empty JSON array and vice-versa.

Tested with Boost 1.92.0 and Visual Studio 2026 on Windows 11.

Start with the minimal std::tuple<> example and trace the value_from and value_to entry points for tuple conversion. Add a regression test covering conversion to an empty JSON array and back to std::tuple<>, then confirm the example compiles and round-trips successfully.

Written by the indexing model from the issue text.