fix: Fix rolling on empty DataFrame panicking

crates/polars-time/src/windows/group_by.rs

@@ -613,6 +613,10 @@ pub fn group_by_values(
     tu: TimeUnit,
     tz: Option<Tz>,
 ) -> PolarsResult<GroupsSlice> {
+    if time.is_empty() {
+        return Ok(GroupsSlice::from(vec![]));
+    }
+
     let mut thread_offsets = _split_offsets(time.len(), POOL.current_num_threads());
     // there are duplicates in the splits, so we opt for a single partition
     prune_splits_on_duplicates(time, &mut thread_offsets);

py-polars/tests/unit/operations/rolling/test_rolling.py

@@ -1260,3 +1260,17 @@ def test_window_size_validation() -> None:
 
     with pytest.raises(OverflowError, match=r"can't convert negative int to unsigned"):
         df.with_columns(trailing_min=pl.col("x").rolling_min(window_size=-3))
+
+
+def test_rolling_empty_21032() -> None:
+    df = pl.DataFrame(schema={"a": pl.Datetime("ms"), "b": pl.Int64()})
+
+    result = df.rolling(index_column="a", period=timedelta(days=2)).agg(
+        pl.col("b").sum()
+    )
+    assert_frame_equal(result, df)
+
+    result = df.rolling(
+        index_column="a", period=timedelta(days=2), offset=timedelta(days=3)
+    ).agg(pl.col("b").sum())
+    assert_frame_equal(result, df)