Awaiting exec::async_scope::on_empty from stdexec::task doesn't compile

[maxbachmann]

stdexec::task<void> test(exec::async_scope& scope)
{
    co_await scope.on_empty();
}

currently doesn't compile because it doesn't find the completion signature

__get_completion_signatures.hpp:62:84: error: invalid use of void expression
   62 |     STDEXEC_PP_FRONT(__VA_ARGS__))::template get_completion_signatures<__VA_ARGS__>()
__get_completion_signatures.hpp:66:5: note: in definition of macro 'STDEXEC_CHECKED_COMPLSIGS'
   66 |     __VA_ARGS__, static_cast<__mlist<STDEXEC_PP_EXPAND _ARGS>*>(nullptr))
      |     ^~~~~~~~~~~
__get_completion_signatures.hpp:228:59: note: in expansion of macro 'STDEXEC_GET_COMPLSIGS'
  228 |         return STDEXEC_CHECKED_COMPLSIGS((_Sender, _Env), STDEXEC_GET_COMPLSIGS(_Sender, _Env));
      |                                                           ^~~~~~~~~~~~~~~~~~~~~