fix: leave lead and lag null_treatment unset by default

Defaulting to NullTreatment.RESPECT_NULLS passed Some(RespectNulls) to
Rust, which adds "RESPECT NULLS" to the generated column name and breaks
code that refers to an un-aliased lead/lag output by name. Default to
None instead; respecting nulls is already the behavior when unset.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: timsaucer

python/datafusion/functions/__init__.py

@@ -7000,7 +7000,7 @@ def lead(
     default_value: Any | None = None,
     partition_by: list[Expr] | Expr | None = None,
     order_by: list[SortKey] | SortKey | None = None,
-    null_treatment: NullTreatment = NullTreatment.RESPECT_NULLS,
+    null_treatment: NullTreatment | None = None,
 ) -> Expr:
     """Create a lead window function.
 
@@ -7079,7 +7079,9 @@ def lead(
             default_value,
             partition_by=partition_by_raw,
             order_by=order_by_raw,
-            null_treatment=null_treatment.value,
+            null_treatment=(
+                null_treatment.value if null_treatment is not None else None
+            ),
         )
     )
 
@@ -7090,7 +7092,7 @@ def lag(
     default_value: Any | None = None,
     partition_by: list[Expr] | Expr | None = None,
     order_by: list[SortKey] | SortKey | None = None,
-    null_treatment: NullTreatment = NullTreatment.RESPECT_NULLS,
+    null_treatment: NullTreatment | None = None,
 ) -> Expr:
     """Create a lag window function.
 
@@ -7166,7 +7168,9 @@ def lag(
             default_value,
             partition_by=partition_by_raw,
             order_by=order_by_raw,
-            null_treatment=null_treatment.value,
+            null_treatment=(
+                null_treatment.value if null_treatment is not None else None
+            ),
         )
     )
 

python/tests/test_dataframe.py

@@ -1186,6 +1186,14 @@ def test_window_partition_by_accepts_string(partitioned_df, partition):
     assert table.column("fv").to_pylist() == [1, 1, 1, 1, 5, 5, 5]
 
 
+@pytest.mark.parametrize("func", [f.lead, f.lag])
+def test_lead_lag_default_null_treatment_keeps_column_name(partitioned_df, func):
+    """Omitting null_treatment must not add RESPECT NULLS to the output name."""
+    df = partitioned_df.select(func(column("b"), order_by=column("a")))
+    name = df.schema().names[0]
+    assert "RESPECT NULLS" not in name
+
+
 @pytest.mark.parametrize(
     ("units", "start_bound", "end_bound"),
     [