Item_ref::val_(datetime|time)_packed() erroneously called
(*ref)->val_(datetime|time)_packed().

- Fixing to call (*ref)->val_(datetime|time)_packed_result().
- Backporting Item::val_(datetime|time)_packed_result() from 10.3.
- Fixing Item_field::get_date_result() to handle null_value in the same
  way how Item_field::get_date() does.