Fixed an issue with an odd reddit api response.

For some odd reason, when attempting to retrieve messages in the category `RKMessageCategoryAll`, the reddit api would respond with `"{}"` if the user has not authenticated yet. This was crashing in `objectsFromListingResponse`, because that response was parsed into an `NSString` by the json parser. I added a catch for this specific response to skip parsing and return nil. Let me know if you think this is an acceptable solution or something else should be done.