Fixing the 5.5 part (the 5.6 part will go in a separate commit soon).

Problem:
  Item_direct_ref::get_date() incorrectly calculated its "null_value",
  which made UNIX_TIMESTAMP(view_column) incorrectly return NULL
  for a NOT NULL view_column.

Fix:
  Make Item_direct_ref::get_date() calculate null_value
  in the similar way with the other methods 
  (val_real,val_str,val_int,val_decimal):
  copy null_value from the referenced Item.

modified:
  mysql-test/r/func_time.result
  mysql-test/t/func_time.test
  sql/item.cc