This fixed failing test in group_by.test

mysql-test/r/join_outer.result:
  Updated test case
mysql-test/r/join_outer_jcl6.result:
  Updated test case
sql/item.cc:
  Don't reset maybe_null in update_used_tables(); This breaks ROLLUP
sql/item.h:
  Don't reset maybe_null in update_used_tables(); This breaks ROLLUP
sql/item_cmpfunc.h:
  Don't reset maybe_null in update_used_tables(); This breaks ROLLUP