fix a bug of const union query like `select null union select null`, this because the type of SlotDescriptor when clause is `select null` is null ,this will cause BE core dump, and FE find wrong cast function.
fix a bug of const union query like `select null union select null`, this because the type of SlotDescriptor when clause is `select null` is null ,this will cause BE core dump, and FE find wrong cast function.