Fix temporary solution for handling foreign SQS messages

To allow handling of foreign SQS messages (not produced by Cheddar) add a single handler for messages of type "" (empty string). This is a temporary solution; the message listener will be generalised at a later time to better decouple message listening and handling.

Signed-off-by: Steffan Westcott <steffan.westcott@clicktravel.com>