Utilise defaultdicts

This code originates from the Python 2.4 supporting version of Flask,
with defaultdicts being added in 2.5. Using defaultdict makes the
intentional usage clearer, and slightly simplifies the code.

src/flask/blueprints.py

@@ -251,7 +251,7 @@ class Blueprint(Scaffold):
             """
             for key, values in self_dict.items():
                 key = self.name if key is None else f"{self.name}.{key}"
-                app_dict.setdefault(key, []).extend(values)
+                app_dict[key].extend(values)
 
         def merge_dict_nested(self_dict, app_dict):
             """Merges self_dict into app_dict. Replaces None keys with self.name.