fix: prevent ambiguous call by calling signal handler with std::nullopt

[ofacklam]

This fixes the following compilation error:

/sdbus-cpp/include/sdbus-c++/ConvenienceApiClasses.inl:465:29: error: call of overloaded ‘apply(const sdbus::test::TestProxy::TestProxy(sdbus::IConnection&, sdbus::ServiceName, sdbus::ObjectPath)::<lambda(std::optional<sdbus::Error>, int)>&, <brace-enclosed initializer list>, sdbus::tuple_of_function_input_arg_types_t<sdbus::test::TestProxy::TestProxy(sdbus::IConnection&, sdbus::ServiceName, sdbus::ObjectPath)::<lambda(std::optional<sdbus::Error>, int)> >&)’ is ambiguous
  465 |                 sdbus::apply(callback, {}, signalArgs);
      |                 ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/sdbus-cpp/include/sdbus-c++/TypeTraits.h:682:30: note: candidate: ‘constexpr decltype(auto) sdbus::apply(Function&&, Result<Args ...>&&, Tuple&&) [with Function = const test::TestProxy::TestProxy(sdbus::IConnection&, sdbus::ServiceName, sdbus::ObjectPath)::<lambda(std::optional<Error>, int)>&; Tuple = std::tuple<int>&; Args = {}]’
  682 |     constexpr decltype(auto) apply(Function&& fun, Result<Args...>&& res, Tuple&& tuple)
      |                              ^~~~~
/sdbus-cpp/include/sdbus-c++/TypeTraits.h:693:20: note: candidate: ‘decltype(auto) sdbus::apply(Function&&, std::optional<Error>, Tuple&&) [with Function = const test::TestProxy::TestProxy(sdbus::IConnection&, sdbus::ServiceName, sdbus::ObjectPath)::<lambda(std::optional<Error>, int)>&; Tuple = std::tuple<int>&]’
  693 |     decltype(auto) apply(Function&& fun, std::optional<Error> err, Tuple&& tuple)
      |                    ^~~~~

[sangelovic]

Good catch. Thanks for fixing this, and for adding the corresponding test case, @ofacklam