from_dicts() entered unreachable code when Categorical column's first value is null, then non-null later

[carsonyl]

Polars version checks

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

Issue description

When using from_dicts() and specifying a Categorical column via schema=, it's possible to encounter pyo3_runtime.PanicException: internal error: entered unreachable code. This happens if the Categorical column's first value is null, and a subsequent row from the list of dicts has a non-null value for it.

This error doesn't happen when the Categorical column starts with a non-null value, or when the column type is instead Utf8.

Reproducible example

import polars as pl
pl.from_dicts(
    [
        {"A": None},
        {"A": "foo"},
    ],
    schema={
        "A": pl.Categorical
    },
)
# pyo3_runtime.PanicException: internal error: entered unreachable code

Expected behavior

The output of the example above should be equal to DataFrame([Series("A", [None, "foo"], dtype=Categorical)]).

Installed versions

---Version info---
Polars: 0.16.8
Index type: UInt32
Platform: Windows-10-10.0.22621-SP0
Python: 3.11.2 (tags/v3.11.2:878ead1, Feb  7 2023, 16:38:35) [MSC v.1934 64 bit (AMD64)]
---Optional dependencies---
pyarrow: 11.0.0
pandas: 1.5.3
numpy: 1.24.2
fsspec: <not installed>
connectorx: <not installed>
xlsx2csv: <not installed>
deltalake: <not installed>
matplotlib: <not installed>

[jonashaag]

This bug is still present in today's main.