fix oob in sorted groupby (#3681)

polars/polars-core/src/frame/groupby/into_groups.rs

@@ -75,6 +75,9 @@ where
             eprintln!("groupby keys are sorted; running sorted key fast path");
         }
         let arr = self.downcast_iter().next().unwrap();
+        if arr.is_empty() {
+            return GroupsSlice::default();
+        }
         let mut values = arr.values().as_slice();
         let null_count = arr.null_count();
 

py-polars/tests/test_groupby.py

@@ -0,0 +1,17 @@
+import polars as pl
+
+
+def test_groupby_sorted_empty_dataframe_3680() -> None:
+    assert (
+        pl.DataFrame(
+            [
+                pl.Series("key", [], dtype=pl.Categorical),
+                pl.Series("val", [], dtype=pl.Float64),
+            ]
+        )
+        .lazy()
+        .sort("key")
+        .groupby("key")
+        .tail(1)
+        .collect()
+    ).shape == (0, 2)