Added a test case for bug #11142.
item_cmpfunc.cc:
  Fixed bug #11142.
  Implementation of Item_func_nullif::is_null was corrected.


sql/item_cmpfunc.cc:
  Fixed bug #11142.
  Implementation of Item_func_nullif::is_null was corrected.
mysql-test/t/func_if.test:
  Added a test case for bug #11142.
mysql-test/r/func_if.result:
  Added a test case for bug #11142.