Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.


sql/sql_class.cc:
  Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.
mysql-test/r/ps_1general.result:
  Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.
mysql-test/r/ps_2myisam.result:
  Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.
mysql-test/r/ps_3innodb.result:
  Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.
mysql-test/r/ps_4heap.result:
  Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.
mysql-test/r/ps_5merge.result:
  Fixed bug #9899: erronious NOT_NULL flag for some attributes
  in the EXPLAIN table.