feat: make auth_id nullable to sync it with the Node implementation

Author: exaby73

src/firebase_functions/firestore_fn.py

@@ -95,7 +95,7 @@ class Event(_core.CloudEvent[_core.T]):
 class AuthEvent(Event[_core.T]):
     auth_type: AuthType
     """The type of principal that triggered the event"""
-    auth_id: str
+    auth_id: str | None
     """The unique identifier for the principal"""