fix: Fix filter scalar nulls (#19786)

crates/polars-core/src/series/implementations/null.rs

@@ -280,6 +280,11 @@ impl SeriesTrait for NullChunked {
             // We still allow a length of `1` because it could be `lit(true)`.
             polars_ensure!(filter.len() <= 1, ShapeMismatch: "filter's length: {} differs from that of the series: 0", filter.len());
             0
+        } else if filter.len() == 1 {
+            return match filter.get(0) {
+                Some(true) => Ok(self.clone().into_series()),
+                None | Some(false) => Ok(NullChunked::new(self.name.clone(), 0).into_series()),
+            };
         } else {
             polars_ensure!(filter.len() == self.len(), ShapeMismatch: "filter's length: {} differs from that of the series: {}", filter.len(), self.len());
             filter.sum().unwrap_or(0) as usize

py-polars/tests/unit/operations/test_filter.py

@@ -298,3 +298,8 @@ def test_invalid_filter_18295() -> None:
             .tail(3)
             .filter(pl.col("value") > 0),
         ).sort("code")
+
+
+def test_filter_19771() -> None:
+    q = pl.LazyFrame({"a": [None, None]})
+    assert q.filter(pl.lit(True)).collect()["a"].to_list() == [None, None]