Empty subscriptions should be disallowed

[ralight]

Hello,

In trying to debug eclipse/mosquitto#1771 I was using MQTT Explorer and inadvertently added an empty subscription to my broker connection. When trying to connect, the empty subscription was correctly rejected by Mosquitto, and an infinite loop of reconnecting occurred. Could you please make it so that subscriptions with empty topic filters are not allowed, or at least are not sent on to the broker?

Thanks!