UNION ALL with nulls can lead to 'Block structure mismatch'

query
```
SELECT
NULL,
NULL
UNION ALL
SELECT
'xxx',
NULL
```

exception
```
Received exception from server (version 23.12.1):
Code: 352. DB::Exception: Received from localhost:9000. DB::Exception: Block structure mismatch in (columns with identical name must have identical structure) stream: different types:
NULL Nullable(String) Nullable(size = 0, String(size = 0), UInt8(size = 0))
NULL Nullable(Nothing) Const(size = 0, Nullable(size = 1, Nothing(size = 1), UInt8(size = 1))). (AMBIGUOUS_COLUMN_NAME)
(query: SELECT
NULL,
NULL
UNION ALL
SELECT
'xxx',
NULL
)
```

https://fiddle.clickhouse.com/278a40b0-80e6-49c5-9c9d-9e704773b917