FIX: create_flat_list: make argument optional

src/caoscrawler/crawl.py

@@ -555,7 +555,7 @@ class Crawler(object):
         return False
 
     @staticmethod
-    def create_flat_list(ent_list: list[db.Entity], flat: Optional[set[db.Entity]]):
+    def create_flat_list(ent_list: list[db.Entity], flat: Optional[set[db.Entity]] = None):
         """
         Recursively adds entities and all their properties contained in ent_list to
         the output set flat.