Invalid Value Offsets Slice For Empty Variable Length Arrays

[tustvold]

Describe the bug

I don't know if this was a change or it has always been this way, but we permit zero-length ListArray to have no offsets. In #1620 we explicitly decided against enforcing a single value.

Unfortunately the logic for value_offsets, duplicated in ListArray, StringArray, BinaryArray, etc...

unsafe {
    std::slice::from_raw_parts(
        self.value_offsets.as_ptr().add(self.data.offset()),
        self.len() + 1,
    )
}

Does not take this into account

To Reproduce

#[test]
fn test_string_array_empty_offsets() {
    let data = ArrayDataBuilder::new(DataType::Utf8)
        .len(0)
        .add_buffer(Buffer::from([]))
        .add_buffer(Buffer::from([]))
        .build()
        .unwrap();
    let array = GenericStringArray::<i32>::from(data);
    assert_eq!(array.value_offsets().len(), 0); // FAILS with 1 != 0
}

Expected behavior

Given the decision to support empty offsets, I think we should always return an empty slice for zero length array types, so that the behaviour is consistent.

Additional context

Discovered whilst working on #1811, so already getting returns from that investment 😁