Don't call defaultHandler if it's not set
Don't call defaultHandler if it's not set

ivan4th/org.eclipse.paho.mqtt.golang,bcbroussard/mqtt-go,ivan4th/org.eclipse.paho.mqtt.golang,ivan4th/org.eclipse.paho.mqtt.golang,choonkeat/paho.mqtt.golang,bcbroussard/mqtt-go,choonkeat/paho.mqtt.golang,cwandrews/mqtt,bcbroussard/mqtt-go,cwandrews/mqtt,cwandrews/mqtt,choonkeat/paho.mqtt.golang