Fixes also:
MDEV-24942 Server crashes in _ma_rec_pack... with DEFAULT() on BLOB

This was caused by two different bugs, both related to that the default
value for the blob was not calculated before it was used:
- There where now Item_default_value::..result() wrappers, which is
  needed as item in HAVING uses these.  This causes crashes when
  using a reference to a DEFAULT(blob_field) in HAVING. It also
  caused wrong results when used with other fields with default value
  expressions that are not constants.
- create_tmp_field() did not take into account that blob fields with
  default expressions are not yet initialized. Fixed by treating
  Item_default_value(blob) like a normal item expression.