Added test cases for bug #11469.
item_strfunc.h:
  Fixed bug #11469: wrong implementation of the not_null_tables
  method for CONCAT_WS.


sql/item_strfunc.h:
  Fixed bug #11469: wrong implementation of the not_null_tables
  method for CONCAT_WS.
mysql-test/t/func_str.test:
  Added test cases for bug #11469.