When do we actually respond with an empty string?

I've been searching through the codebase, and I do not see how this could happen with the `HubDispatcher`. The only way I can see this happening is if someone creates a `PersistentConnection` without calling `Transport.Send` in `OnReceived`. This is probably pretty common for `PersistentConnection`'s. `RawConnection` in our sample project doesn't do this.

The good news is that if `Transport.Send` isn't called, the JSONP callback will never be called with an empty string or otherwise, so that would explain why our testers didn't see any errors with JSONP.