fix: Add unique fast path for empty categoricals (#20536)

crates/polars-core/src/chunked_array/logical/categorical/ops/unique.rs

@@ -7,6 +7,18 @@ use super::*;
 impl CategoricalChunked {
     pub fn unique(&self) -> PolarsResult<Self> {
         let cat_map = self.get_rev_map();
+        if self.is_empty() {
+            // SAFETY: rev map is valid.
+            unsafe {
+                return Ok(CategoricalChunked::from_cats_and_rev_map_unchecked(
+                    UInt32Chunked::full_null(self.name().clone(), 0),
+                    cat_map.clone(),
+                    self.is_enum(),
+                    self.get_ordering(),
+                ));
+            }
+        };
+
         if self._can_fast_unique() {
             let ca = match &**cat_map {
                 RevMapping::Local(a, _) => UInt32Chunked::from_iter_values(

py-polars/tests/unit/operations/unique/test_unique.py

@@ -154,6 +154,16 @@ def test_unique_categorical(input: list[str | None], output: list[str | None]) -
     assert_series_equal(result, expected)
 
 
+def test_unique_categorical_global() -> None:
+    with pl.StringCache():
+        pl.Series(["aaaa", "bbbb", "cccc"])  # pre-fill global cache
+        s = pl.Series(["a", "b", "c"], dtype=pl.Categorical)
+        s_empty = s.slice(0, 0)
+
+        assert s_empty.unique().to_list() == []
+        assert_series_equal(s_empty.cat.get_categories(), pl.Series(["a", "b", "c"]))
+
+
 def test_unique_with_null() -> None:
     df = pl.DataFrame(
         {