fix(rust, python): object filter should keep single chunk (#5913)

polars/polars-core/src/chunked_array/object/builder.rs

@@ -142,4 +142,8 @@ where
             length: len as IdxSize,
         }
     }
+
+    pub fn new_empty(name: &str) -> Self {
+        Self::new_from_vec(name, vec![])
+    }
 }

polars/polars-core/src/chunked_array/ops/filter.rs

@@ -157,7 +157,7 @@ where
         if filter.len() == 1 {
             return match filter.get(0) {
                 Some(true) => Ok(self.clone()),
-                _ => Ok(ObjectChunked::from_chunks(self.name(), vec![])),
+                _ => Ok(ObjectChunked::new_empty(self.name())),
             };
         }
         if self.is_empty() {

py-polars/tests/unit/test_object.py

@@ -15,3 +15,22 @@ def test_object_when_then_4702() -> None:
         "Type": [pl.Date, pl.UInt8],
         "New_Type": [pl.UInt16, pl.UInt8],
     }
+
+
+def test_object_empty_filter_5911() -> None:
+    df = pl.DataFrame(
+        data=[
+            (1, "dog", {}),
+        ],
+        columns=[
+            ("pet_id", pl.Int64),
+            ("pet_type", pl.Categorical),
+            ("pet_obj", pl.Object),
+        ],
+        orient="row",
+    )
+
+    empty_df = df.filter(pl.col("pet_type") == "cat")
+    out = empty_df.select(["pet_obj"])
+    assert out.dtypes == [pl.Object]
+    assert out.shape == (0, 1)