replace_time_zone with ambiguous with single null value panics

[MarcoGorelli]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl
from datetime import datetime
df = pl.DataFrame(
    {
        "ts": [datetime(2020, 10, 2, 1, 1)],
        "ambiguous": [None],
    },
    schema_overrides={'ambiguous': pl.String}
)

result = df.select(
    pl.col("ts").dt.replace_time_zone(
        "Europe/London", ambiguous=pl.col("ambiguous")
    )
)["ts"]
print(result)

Log output

thread '<unnamed>' panicked at crates/polars-ops/src/chunked_array/datetime/replace_time_zone.rs:71:76:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Traceback (most recent call last):
  File "/home/marcogorelli/scratch/t.py", line 11, in <module>
    result = df.select(
             ^^^^^^^^^^
  File "/home/marcogorelli/scratch/.venv/lib/python3.11/site-packages/polars/dataframe/frame.py", line 8116, in select
    return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/marcogorelli/scratch/.venv/lib/python3.11/site-packages/polars/lazyframe/frame.py", line 1934, in collect
    return wrap_df(ldf.collect())
                   ^^^^^^^^^^^^^
pyo3_runtime.PanicException: called `Option::unwrap()` on a `None` value

Issue description

it panics

Expected behavior

it should just return a null

Installed versions

--------Version info---------
Polars:               0.20.14
Index type:           UInt32
Platform:             Linux-5.15.146.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:               3.11.7 (main, Dec  8 2023, 18:56:58) [GCC 11.4.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               <not installed>
gevent:               <not installed>
hvplot:               0.9.2
matplotlib:           <not installed>
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.1
pyarrow:              15.0.0
pydantic:             2.6.1
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>```

</details>