Describe the bug

During testing of the initial implementation of FFI user defined window functions, I generated a panic in partition_evaluator.

To Reproduce

Open examples/datafusion-ffi-example/src/window_udf.rs

Change this to use lead_udwf() instead of rank_udwf().

Run integration test in examples/datafusion-ffi-example/python/tests/_test_window_udf.py

Expected behavior

Should not panic.

Additional context

It is unclear if the problem is in datafusion-python, or more likely upstream.