The Item_func_mod objects never had maybe_null set, so users had no reason
to expect that they can be NULL, and may therefore deduce wrong results.
Now, set maybe_null.
mysql-test/r/func_test.result:
Verify that the predictions are true.
mysql-test/t/func_test.test:
Verify that the predictions are true.
sql/item_func.cc:
MOD functions may be NULL.
