USED
Issue :
-------
Temporal literals which are in ODBC STYLE are considered as
data/time constants. For such items we have used_tables as 0
but did not initialize not_null_tables. not_null_tables is
now pointing to some junk table_map values. While in
simplify_joins the conditions with temporal literals were
wrongly assumed to filter nulls in inner table of outer
joins. This is because not_null_tables is not initialized.
So some of the outer joins are converted to inner joins.
This results in invalid result row set.

Solution:
---------
For class Item_date_literal, Item_time_literal and
Item_datetime_literal set not_null_tables as used_tables().