Bug #33629: last_day function can return null, but has 'not null' flag set for result

LAST_DAY and MAKEDATE functions are documented as
returning NULL value, but actually they was implemented
as returning NOT NULL typed values.

That caused a confusing error "ERROR 1048 (23000): Column
'...' cannot be null" on queries like: 

  SELECT 1 FROM (SELECT LAST_DAY('0')) a;


mysql-test/r/func_sapdb.result:
    Updated test case for bug #33629.
mysql-test/r/func_time.result:
    Updated test case for bug #33629.
mysql-test/r/type_date.result:
    Added test case for bug #33629.
mysql-test/t/type_date.test:
    Added test case for bug #33629.
sql/item_timefunc.h:
    Bug #33629: last_day function can return null, but has 'not null' flag set for result
    
    1. The Item_func_makedate::fix_length_and_dec method
       has been modified to declare MAKEDATE() as a function
       returning nullable value.
    2. The Item_func_last_day::fix_length_and_dec method
       has been overloaded for the same purpose.