Exploding list of structs fails in some cases

[indigoviolet]

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• I have confirmed this bug exists on the latest version of Polars.

Issue description

Exploding a series of List[struct] containing a list field fails in some situations, in an order-dependent fashion. It is also unclear why a list[struct] would explode to a null-struct (see below)

Reproducible example

import polars as pr
pr.Series(
    [[], [{"b": []}, {"b": ["foo"]}]],
    dtype=pr.List(pr.Struct({"b": pr.List(pr.Utf8)})),
).explode()
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: OutOfSpec("validity mask length must match the number of values")', /root/.cargo/git/checkouts/arrow2-8a2ad61d97265680/368aacc/src/array/list/mod.rs:202:71

Why does the [] row explode to {null}?

import polars as pr
pr.Series(
    [[{"b": []}, {"b": ["foo"]}], []`],
    dtype=pr.List(pr.Struct({"b": pr.List(pr.Utf8)})),
).explode()

pr.Series(
    [[{"b": []}, {"b": ["foo"]}], []],
    dtype=pr.List(pr.Struct({"b": pr.List(pr.Utf8)})),
).explode()
Out [230]:
shape: (3,)
Series: '' [struct[1]]
[
	{[]}
	{["foo"]}
	{null}
]

Expected behavior

I would expect the explode() to work, and for [] to explode to no rows

Installed versions

---Version info---
Polars: 0.15.1
Index type: UInt32
Platform: Linux-4.19.0-22-cloud-amd64-x86_64-with-glibc2.10
Python: 3.8.13 | packaged by conda-forge | (default, Mar 25 2022, 06:04:10) 
[GCC 10.3.0]
---Optional dependencies---
pyarrow: 7.0.1
pandas: 1.5.1
numpy: 1.23.4
fsspec: 2022.10.0
connectorx: <not installed>
xlsx2csv: <not installed>
matplotlib: 3.6.2

[ritchie46]

Thanks, will take a look. BTW could you use pl instead of pr? That's how we use it in every example and it really saves me a lot of muscle memory rewiring.