Fix persistent index creation signature (#229)

Author: aquamatthias

arango/collection.py

@@ -1248,7 +1248,7 @@ def add_persistent_index(
         sparse: Optional[bool] = None,
         name: Optional[str] = None,
         in_background: Optional[bool] = None,
-        storedValues: Sequence[str] = [],
+        storedValues: Optional[Sequence[str]] = None,
         cacheEnabled: Optional[bool] = None,
     ) -> Result[Json]:
         """Create a new persistent index.