improve rolling_agg (#3101)

handle NaNs with IsFloat trait

polars/polars-arrow/src/data_types.rs

@@ -0,0 +1,39 @@
+pub trait IsFloat {
+    fn is_float() -> bool {
+        false
+    }
+
+    #[allow(clippy::wrong_self_convention)]
+    fn is_nan(self) -> bool
+    where
+        Self: Sized,
+    {
+        false
+    }
+}
+
+impl IsFloat for i8 {}
+impl IsFloat for i16 {}
+impl IsFloat for i32 {}
+impl IsFloat for i64 {}
+impl IsFloat for u8 {}
+impl IsFloat for u16 {}
+impl IsFloat for u32 {}
+impl IsFloat for u64 {}
+
+macro_rules! impl_is_float {
+    ($tp:ty) => {
+        impl IsFloat for $tp {
+            fn is_float() -> bool {
+                true
+            }
+
+            fn is_nan(self) -> bool {
+                <$tp>::is_nan(self)
+            }
+        }
+    };
+}
+
+impl_is_float!(f32);
+impl_is_float!(f64);