Use dummy UInt8 for default structure of table function null

Why it matters

Before this change, the null table function had an inconsistent or unspecified default structure, which could cause issues in queries expecting a predictable schema. Using dummy UInt8 as the default column ensures a consistent and clear schema, making it easier for users to work with null tables in their queries.

How to use it

When using the null table function without specifying columns, it will automatically return a table with a single dummy column of type UInt8. No additional configuration is needed; this change is applied internally.