std::valarray lacks empty method. Even stranger is that std::empty added in
C++17 has no overload for std::valarray, but it has an overload for
std::initializer_list.I think the empty method does not just belong to the
containers, they are also useful for std::valarray because it should be
normal to check if a numeric array is empty. Therefore, empty method should
be added to std::valarray.