Added a test case for bug #8669.
item_strfunc.cc:
  Fixed bug #8669.
  Function AES_DECRYPT can return NULL value.


sql/item_strfunc.cc:
  Fixed bug #8669.
  Function AES_DECRYPT can return NULL value.
mysql-test/t/func_str.test:
  Added a test case for bug #8669.
mysql-test/r/func_str.result:
  Added a test case for bug #8669.