Add test_aggregation_empty

chinese_whispers/test_chinese_whispers.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from random import Random
-from typing import TYPE_CHECKING, cast
+from typing import TYPE_CHECKING, Any, cast
 
 import networkx as nx
 import pytest
@@ -98,6 +98,12 @@ def test_ignore_one(graph: nx.Graph[int]) -> None:
     assert set(graph) - nodes == ignore
 
 
+def test_aggregation_empty() -> None:
+    clusters: dict[int, set[Any]] = aggregate_clusters(nx.Graph())
+
+    assert not clusters
+
+
 def test_aggregation(clustered_graph: nx.Graph[int]) -> None:
     clusters = aggregate_clusters(clustered_graph)