dvizz fails to start if no service is running

[liske]

Starting dvizz on a docker swarm w/o any running service will result in a panic:

$ docker run -v /var/run/docker.sock:/var/run/docker.sock -p 6969:6969 -it --rm liske/dvizz
Starting WebSocket server at port 6969
Starting WebSocket server
Starting event sender goroutine...
panic: interface conversion: interface {} is nil, not []main.DService

goroutine 8 [running]:
main.convServices(0xa98588, 0x0, 0x0, 0x0, 0xa98588, 0x0)
        /dvizz/converters.go:67 +0xe0
main.publishServices(0xc420114090)
        /dvizz/main.go:108 +0x80
created by main.main
        /dvizz/main.go:50 +0x14e

@eriklupander I'm not a go developer - maybe you could give it a look?

[eriklupander]

@liske I'm on it.

[eriklupander]

I've added some nil and empty-slice checks as well as some unit tests.