Set null_value in case of wrong data.


mysql-test/r/cast.result:
  fix for bug #23938: ISNULL on DATE AND CAST AS DATE returns false for null values
    - test result.
mysql-test/t/cast.test:
  fix for bug #23938: ISNULL on DATE AND CAST AS DATE returns false for null values
    - test case.
sql/item_timefunc.cc:
  fix for bug #23938: ISNULL on DATE AND CAST AS DATE returns false for null values
    - set null_value in case of error.