Item_func_equal::not_null_tables() always return 0.


mysql-test/r/join_outer.result:
  Testcase for BUG#8711
mysql-test/t/join_outer.test:
  Testcase for BUG#8711
sql/item_cmpfunc.h:
  Fix for BUG#8711: "<=>" may have true value for NULL arguments, i.e. it can accept NULL-complemented 
  table rows. This differs from assumptions made in Item_func::not_null_tables(), so add 
  Item_func_equal::not_null_tables().
  Item_func_equal::not_null_tables_cache value doesn't make sense now, but we still let it be
  calculated (and ignore it)