Returns `input_sender`, but will make an API call to find the
        input sender unless it's already cached.

async def get_input_sender(self):
        """
        Returns `input_sender`, but will make an API call to find the
        input sender unless it's already cached.
        """
        if self.input_sender is None and self._sender_id:
            await self._refetch_sender()
        return self._input_sender