Omit params for execute if empty

... to avoid the following warning which was added by [1]

```
sqlalchemy.exc.SADeprecationWarning: Empty parameter sequence passed to
execute(). This use is deprecated and will raise an exception in
a future SQLAlchemy release
```

[1] sqlalchemy/sqlalchemy@af655e5

Change-Id: I2dc73c422e38d002717a5f4e0db20c696b4df581

oslo_db/tests/sqlalchemy/test_exc_filters.py

@@ -166,7 +166,7 @@ def do_execute(self, cursor, statement, parameters, **kw):
             yield
 
     def _run_test(self, dialect_name, statement, raises, expected,
-                  is_disconnect=False, params=()):
+                  is_disconnect=False, params=None):
         with self._fixture(dialect_name, raises, is_disconnect=is_disconnect):
             with self.engine.connect() as conn:
                 matched = self.assertRaises(