fix: fix seg fault in concat_str of empty series (#11704)

crates/polars-ops/src/chunked_array/strings/concat.rs

@@ -65,6 +65,11 @@ pub fn hor_str_concat(cas: &[&Utf8Chunked], delimiter: &str) -> PolarsResult<Utf
         ComputeError: "all series in `hor_str_concat` should have equal or unit length"
     );
 
+    let has_empty_ca = cas.iter().any(|ca| ca.is_empty());
+    if has_empty_ca {
+        return Ok(Utf8Chunked::full_null(cas[0].name(), 0));
+    }
+
     // Calculate total capacity needed.
     let tot_strings_bytes: usize = cas
         .iter()

py-polars/tests/unit/functions/test_concat.py

@@ -1,6 +1,7 @@
 import pytest
 
 import polars as pl
+from polars.testing import assert_frame_equal
 
 
 @pytest.mark.slow()
@@ -20,3 +21,9 @@ def test_concat_lf_stack_overflow() -> None:
     for i in range(n):
         bar = pl.concat([bar, pl.DataFrame({"a": i}).lazy()])
     assert bar.collect().shape == (1001, 1)
+
+
+def test_empty_df_concat_str_11701() -> None:
+    df = pl.DataFrame({"a": []})
+    out = df.select(pl.concat_str([pl.col("a").cast(pl.Utf8), pl.lit("x")]))
+    assert_frame_equal(out, pl.DataFrame({"a": []}, schema={"a": pl.Utf8}))