`geoarrow.rust.io.write_flatgeobuf()` panics when writing data that contains nulls

[paleolimbot]

I can follow up with a more complete example, but when preparing geoarrow-data I tried to write the example files, all of which contain nulls. When I try to write such a file I get:

pyo3_runtime.PanicException: called `Option::unwrap()` on a `None` value