fix: improve unnest_generic_list handling of null list

[jonahgao]

Which issue does this PR close?

Closes #9932.

Rationale for this change

In a GenericListArray, the data in the values corresponding to a null list may not be empty; it can be any data.

pub struct GenericListArray<OffsetSize: OffsetSizeTrait> {
    data_type: DataType,
    nulls: Option<NullBuffer>,
    values: ArrayRef,
    value_offsets: OffsetBuffer<OffsetSize>,
}

For example, in the description of this document, the value of the null list is ?, it's not empty.

In this situation, merely checking the values is unsafe, we must obtain valid values based on the value_offsets.

What changes are included in this PR?

• Fix bug in unnest_generic_list.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[Jefffrey]

Looks correct to me 👍

[Jefffrey]

Bit surprised that clippy didn't suggest collapsing these together? Either way, would it be slightly more efficient to put the options.preserve_nulls check first since this would never change per iteration?

Edit: ah nvm, it's because the outer one is an if else 😅

[Jefffrey]

Nice tests 👍

[jayzhan211]

👍

[jayzhan211]

Nice fix! Thanks @jonahgao and @Jefffrey