Allow NULL Modifier in Table Function Column Declarations

Why it matters

This feature enables users to specify columns as NULLable directly within table function declarations. It improves flexibility and data schema expressiveness when using table functions, allowing better handling of optional or missing data.

How to use it

When defining columns in table functions, users can now add the NULL modifier to column types to indicate that the columns accept NULL values. For example:

WITH table_function(...)
( column1 Nullable(Int32), column2 Nullable(String) )

This enables the table function to return columns that support NULLs.