Disallow sending empty messages

[karalabe]

It should be decided whether empty messages make sense or not, but if not, they should be explicitly disallowed and violating connections dropped to prevent faulty API uses.

The current relay protocol spec cannot accept empty tunnel messages, so either a rework is needed there, or just disable empty messages everywhere.

[karalabe]

After sleeping on this a bit, I think I'm going to disallow empty messages. This approach is backward compatible in the sense that it should not require a heavy relay protocol redesign rewrite. It may break code using empty messages, but imho that was already broken by design, so lets correct this here and now and not propagate invalid/ambiguous use of the system.