🐛 Fix tuple_size_v for types which shouldn't have it

[elbeno]

Problem:

• The concept has_tuple_protocol incorrectly succeeds on some platforms for types like std::optional<T> because the primary template definition for stdx::tuple_size_v is:

template <typename T> constexpr auto tuple_size_v = 0u;

And on some platforms, std::size_t is unsigned int, which means literally any type satisfies has_vacuous_tuple_protocol concept!

Solution:

• Make the primary template definition of tuple_size_v an invalid type.