'not exists' optimization is applied.

In fact 'not exists' optimization did not work anymore after the patch
introducing the evaluate_join_record function had been applied.

Corrected the evaluate_join_record function to respect the 'not_exists'
optimization.


mysql-test/r/join_outer.result:
  Added a test case for bug #28188.
mysql-test/t/join_outer.test:
  Added a test case for bug #28188.
sql/sql_select.cc:
  Fixed bug #28188: performance degradation for outer join queries to which
  'not exists' optimization is applied.
  
  Corrected the evaluate_join_record function to respect the 'not_exists'
  optimization.