fix empty union

bin/stats.py

@@ -123,7 +123,8 @@ def problem_stats(problems):
 
         def count(path):
             if type(path) is list:
-                return set.union(*(count(p) for p in path))
+                paths = [*(count(p) for p in path)]
+                return set.union(*paths) if paths else set()
             if callable(path):
                 return path(generated_testcases)
             results: set[str | Path] = set()