fix: replace_time_zone with single-null-element "ambiguous" was panic…

…king

crates/polars-ops/src/chunked_array/datetime/replace_time_zone.rs

@@ -68,7 +68,10 @@ pub fn replace_time_zone(
         }),
     };
     let mut out = out?.into_datetime(datetime.time_unit(), time_zone.map(|x| x.to_string()));
-    if from_time_zone == "UTC" && ambiguous.len() == 1 && ambiguous.get(0).unwrap() == "raise" {
+    if from_time_zone == "UTC"
+        && ambiguous.len() == 1
+        && unsafe { ambiguous.get_unchecked(0) } == Some("raise")
+    {
         // In general, the sortedness flag can't be preserved.
         // To be safe, we only do so in the simplest case when we know for sure that there is no "daylight savings weirdness" going on, i.e.:
         // - `from_tz` is guaranteed to not observe daylight savings time;

py-polars/tests/unit/datatypes/test_temporal.py

@@ -1831,6 +1831,19 @@ def test_ambiguous_expressions() -> None:
     assert_series_equal(result, expected)
 
 
+def test_single_ambiguous_null() -> None:
+    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"].item()
+    assert result is None
+
+
 def test_unlocalize() -> None:
     tz_naive = pl.Series(["2020-01-01 03:00:00"]).str.strptime(pl.Datetime)
     tz_aware = tz_naive.dt.replace_time_zone("UTC").dt.convert_time_zone(