fix: handle None from Redis in sent_messages counter

When Redis keys are missing (after FLUSHALL, restart, or for new
mailboxes/domains), hget() returns None. The sent_messages property
in MessageLimitMixin now treats a missing counter as zero messages
sent, preventing TypeError on /api/v2/accounts/ and /api/v2/domains/.

modoboa/admin/models/mixins.py

@@ -17,7 +17,10 @@ class MessageLimitMixin:
         """Return number of sent messages for the current day."""
         if self.message_limit is None:
             return None
-        return self.message_limit - get_message_counter(self.message_counter_key)
+        counter = get_message_counter(self.message_counter_key)
+        if counter is None:
+            return 0
+        return self.message_limit - counter
 
     @property
     def sent_messages_in_percent(self) -> Optional[int]:  # noqa