`unnest` of a struct ignores the struct's null mask

[neilconway]

Describe the bug

Unnesting a struct column treats the child arrays as independent of the struct's own validity, in both the planner and the executor:

• get_unnested_columns copies each child's declared nullability into the output schema without checking whether the parent struct is nullable.
• flatten_struct_cols returns struct_arr.columns() verbatim, without applying the struct's null buffer.

To Reproduce

This can result in unnesting values from a "null" struct:

-- One row holding a fully valid struct.
create table base as select named_struct('a', 1, 'b', 'x') as s;

-- nullif on a column goes through arrow's nullif kernel, which only flips the
-- struct's validity bit. The child arrays still hold 1 and 'x'.
create table t2 as select nullif(s, s) as s from base;

select s from t2;
-- NULL

select s is null from t2;
-- true

select unnest(s) from t2;
-- 1 x

select unnest(arrow_cast(NULL, 'Struct("a": non-null Int32)'));

yields:

Arrow error: Invalid argument error: Column '__unnest_placeholder(arrow_cast(NULL,Utf8("Struct("a": non-null Int32)"))).a' is declared as non-nullable but contains null values

Expected behavior

• Unnesting a NULL struct row yields NULL for every output column.
• Every output column of a struct unnest is nullable whenever the parent struct is nullable, matching the rule get_field already follows for nested access.

i.e., first query above should return NULL NULL, second query should return NULL instead of an error.

Additional context

Similar to #25120