[Varaint]: add DataType::Null support to cast_to_variant

[feniljain]

Which issue does this PR close?

• Closes [Variant]: Implement DataType::Null support for cast_to_variant kernel #8053

Are these changes tested?

Yes

Notes

From this comment:

/// # Notes
/// If the input array element is null, the corresponding element in the
/// output `VariantArray` will also be null (not `Variant::Null`).

I think we need to make a variant array of nulls, do correct me if I am wrong here, and a Variant::Null needs to be returned 😅

[alamb]

I agree this makes sense to me -- thank you @feniljain

[alamb]

I merged up to resolve a conflict

[alamb]

Thanks again @feniljain