Preserve column hierarchy when getting NULL row from LIST column

[isVoid]

This PR fixes a bug introduced in #8071, when get_element retrieves a NULL row in a nested column, the scalar returned not only should be is_valid() == false, but also should preserve the column hierarchy of the row-data, even they are invalid. Because depending libraries may use the column hierarchy to deduce the nested type of the column.

This PR also reverts make_default_constructed_scalar API for LIST type. A LIST type scalar should have complete column hierarchy as part of its type information. There isn't enough information provided to the API to construct that.

Another tiny addition: instead of hard coding the position of child column, use list_column_view::child_column_index intead.

[codereport]

lgtm

[codecov]

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[isVoid]

Don't know if I have cpp merge privilege so I'll test on this one.
@gpucibot merge

[harrism]

@gpucibot merge

[harrism]

@isVoid You need to put the @gpucibot merge alone in a comment.