`Sparse` serialization should be supported for `Nullable`

**Describe the situation**

```
CREATE TEMPORARY TABLE test (x Nullable(UInt64), PRIMARY KEY ()) ENGINE = MergeTree SETTINGS ratio_of_defaults_for_sparse_serialization = 0;
INSERT INTO test SELECT number % 2 ? number * 123456789 : NULL FROM numbers(10);

SELECT DISTINCT dumpColumnStructure(*) FROM test;
```