Bring back is emtpy for std::optional

Author: filipsajdak

include/cpp2util.h

@@ -1488,11 +1488,9 @@ template<non<void> T, std::same_as<std::optional<T>> X> // same_as needed for cl
 constexpr auto is( X const& x ) -> bool
     { return x.has_value(); }
 
-// handled by pointer_like overload is it good?
-//
-// template<std::same_as<empty> T, non<void> U>
-// constexpr auto is( std::optional<U> const& x ) -> bool
-//     { return !x.has_value(); }
+template<std::same_as<empty> T, non<void> U>
+constexpr auto is( std::optional<U> const& x ) -> bool
+    { return !x.has_value(); }
 
 
 //  is Value