Fix: pick message to delete (#12295)

### What problem does this PR solve?

Pick unforgotten message when not found forgotten message to delete.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

memory/services/messages.py

@@ -194,11 +194,10 @@ class MessageService:
         select_fields = ["message_id", "content", "content_embed"]
         _index_name = index_name(uid)
         res = settings.msgStoreConn.get_forgotten_messages(select_fields, _index_name, memory_id)
-        if not res:
-            return []
-        message_list = settings.msgStoreConn.get_fields(res, select_fields)
         current_size = 0
         ids_to_remove = []
+        if res:
+            message_list = settings.msgStoreConn.get_fields(res, select_fields)
             for message in message_list.values():
                 if current_size < size_to_delete:
                     current_size += cls.calculate_message_size(message)