Return None for nullptr

Author: rhaschke

include/pybind11/detail/smart_holder_type_casters.h

@@ -623,6 +623,9 @@ struct smart_holder_type_caster<std::shared_ptr<T>> : smart_holder_type_caster_l
     static constexpr auto name = _<std::shared_ptr<T>>();
 
     static handle cast(const std::shared_ptr<T> &src, return_value_policy policy, handle parent) {
+        if (!src)
+            return none().release();
+
         auto src_raw_ptr = src.get();
         auto st          = type_caster_base<T>::src_and_type(src_raw_ptr);
         if (st.second == nullptr)
@@ -687,6 +690,8 @@ struct smart_holder_type_caster<std::unique_ptr<T, D>> : smart_holder_type_caste
             // SMART_HOLDER_WIP: IMPROVABLE: Error message.
             throw cast_error("Invalid return_value_policy for unique_ptr.");
         }
+        if (!src)
+            return none().release();
 
         auto src_raw_ptr = src.get();
         auto st          = type_caster_base<T>::src_and_type(src_raw_ptr);