handling null pointers/records to functions

["quasar" <quasar@nospam.com>]

Hi , I was wondering what is the best way to handle null records/pointers 
passed to functions with a return type that is generic?

for example in this snippet of code node_ptr_type is a record holding a 
single data value of type Data_Type

function Data_Part (Node_Ptr : Node_Ptr_Type ) return Data_Type is
begin
    if node_ptr /= null then
        return Node_Ptr.Data;
    else
        Put_Line("Node passed to FUNCTION DATA_PART is null");
        {(**)at this point i do not know what to do... do I terminate the 
program, do I return some form of null object}
    end if;
end Data_Part;

What is the most recommended way to handle this? Point (**) has got me 
stuck.

Thanks any help would be great
quasar