Transform tunnels stall when no bytes are sent

While researching the assert described in issue #8079, we noticed that a WriteTransform plugin applied to a 0 length body will stall. The logic to fully set up the WriteTransform tunnel requires receiving a WRITE_READY event. But in the case of a 0 length body, the tunnel will go directly into READ_COMPLETE, WRITE_COMPLETE, and TUNNEL_PRECOMPLETE. No do_io_write or do_io_read operations will be set up, so no READ_READY/WRITE_READY events will be sent to trigger the sending of the TRANSFORM_READ_READY.

Eventually sometime will timeout and kill the HttpSM.