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;