code: def handle_package_literal_optional(self, package, package_node, predicate, field):
          
          if package.has_optional_field(field):
              value = getattr(package, field, None)
              value_node = self.to_special_value(value)
              triple = (package_node, predicate, value_node)
              self.graph.add(triple)
docs: Check if optional field is set.
          If so it adds the triple (package_node, predicate, $) to the graph.
          Where $ is a literal or special value term of the value of the field.