Fixed NullReferenceException in Oracle non-parameterized update statements

Attempting to call `Db.Update(someObject);` on an Oracle database would cause `OracleOrmLiteDialectProvider.ToUpdateRowStatement()` to throw a NullReferenceException on `updateFields`.

The method was missing the standard if-null-then-use-an-empty-list pattern from elsewhere in OrmLite.

If `Db.UpdateParam(someObject);` was called instead, the DB call would work (since a different dialect method is called, which had the correct pattern).