Error when writing to csv when DataFrame contains both naive and aware datetimes and no datetime_format is specified

[MarcoGorelli]

Polars version checks

• I have checked that this issue has not already been reported.

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

Issue description

I accidentally introduced in #6722 so sorry!

Reproducible example

In [3]: from datetime import datetime
   ...: import datetime as dt
   ...:
   ...: import polars as pl
   ...:
   ...: ts = pl.date_range(datetime(2000, 1, 1), datetime(2000, 1, 2))
   ...: df = pl.DataFrame({"naive": ts, "aware": ts.dt.replace_time_zone("UTC")})
   ...: print(df.write_csv())
   ...:
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Uncategorized, message: "formatter error" }', /home/runner/work/polars/polars/polars/polars-io/src/csv/write_impl.rs:128:6
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
---------------------------------------------------------------------------
PanicException                            Traceback (most recent call last)
Cell In [3], line 9
      7 ts = pl.date_range(datetime(2000, 1, 1), datetime(2001, 1, 1))
      8 df = pl.DataFrame({"naive": ts, "aware": ts.dt.replace_time_zone("UTC")})
----> 9 print(df.write_csv())

File ~/tmp/.venv/lib/python3.8/site-packages/polars/internals/dataframe/frame.py:2295, in DataFrame.write_csv(self, file, has_header, sep, quote, batch_size, datetime_format, date_format, time_format, float_precision, null_value)
   2293 if file is None:
   2294     buffer = BytesIO()
-> 2295     self._df.write_csv(
   2296         buffer,
   2297         has_header,
   2298         ord(sep),
   2299         ord(quote),
   2300         batch_size,
   2301         datetime_format,
   2302         date_format,
   2303         time_format,
   2304         float_precision,
   2305         null_value,
   2306     )
   2307     return str(buffer.getvalue(), encoding="utf-8")
   2309 if isinstance(file, (str, Path)):

PanicException: called `Result::unwrap()` on an `Err` value: Error { kind: Uncategorized, message: "formatter error" }

Expected behavior

naive,aware
2000-01-01T00:00:00.000000,2000-01-01T00:00:00.000000+0000
2000-01-02T00:00:00.000000,2000-01-02T00:00:00.000000+0000

Installed versions

---Version info---
Polars: 0.16.4
Index type: UInt32
Platform: Linux-5.10.102.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python: 3.10.6 (main, Nov 14 2022, 16:10:14) [GCC 11.3.0]
---Optional dependencies---
pyarrow: 11.0.0
pandas: 1.5.3
numpy: 1.24.1
fsspec: <not installed>
connectorx: <not installed>
xlsx2csv: 0.8.1
deltalake: <version not detected>
matplotlib: <not installed>
None

[MarcoGorelli]

PR on its way