Fix Webhook poll sending raising AttributeError with a mocked state

discord/webhook/async_.py

@@ -510,6 +510,11 @@ class _WebhookState:
             return self._parent._get_guild(guild_id)
         return None
 
+    def _get_poll(self, msg_id: Optional[int]) -> Optional[Poll]:
+        if self._parent is not None:
+            return self._parent._get_poll(msg_id)
+        return None
+
     def store_user(self, data: Union[UserPayload, PartialUserPayload], *, cache: bool = True) -> BaseUser:
         if self._parent is not None:
             return self._parent.store_user(data, cache=cache)