fix: Adding a struct field using a literal raises InvalidOperationErr…

…or (#21254)

crates/polars-plan/src/dsl/function_expr/struct_.rs

@@ -245,8 +245,7 @@ pub(super) fn with_fields(args: &[Column]) -> PolarsResult<Column> {
     }
 
     let new_fields = fields.into_values().cloned().collect::<Vec<_>>();
-    let mut out =
-        StructChunked::from_series(ca.name().clone(), new_fields[0].len(), new_fields.iter())?;
+    let mut out = StructChunked::from_series(ca.name().clone(), ca.len(), new_fields.iter())?;
     out.zip_outer_validity(ca);
     Ok(out.into_column())
 }

py-polars/tests/unit/datatypes/test_struct.py

@@ -1227,6 +1227,18 @@ def test_nested_object_raises_15237() -> None:
         df.select(pl.struct("a"))
 
 
+def test_empty_struct_with_fields_21095() -> None:
+    df = pl.DataFrame({"a": [{}, {}]})
+    assert_frame_equal(
+        df.select(pl.col("a").struct.with_fields(a=pl.lit(42, pl.Int64))),
+        pl.DataFrame({"a": [{"a": 42}, {"a": 42}]}),
+    )
+    assert_frame_equal(
+        df.select(pl.col("a").struct.with_fields(a=None)),
+        pl.DataFrame({"a": [{"a": None}, {"a": None}]}),
+    )
+
+
 def test_cast_to_struct_needs_field_14083() -> None:
     with pytest.raises(
         InvalidOperationError, match="must specify one field in the struct"