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.