Make sure app does not crash when webirc is not defined in the configuration

[astorije]

The check currently tests against null. When the configuration file does not even have webirc, its value is undefined, not null:

/home/dusk/lounge/src/client.js:175
        if (config.webirc !== null && network.host in config.webirc) {
                                                            ^

TypeError: Cannot use 'in' operator to search for 'ocelot.link' in undefined
    at Client.connect ([...]/lounge/src/client.js:175:54)
    at Timeout._onTimeout ([...]/lounge/src/client.js:73:12)
    at tryOnTimeout (timers.js:224:11)
    at Timer.listOnTimeout (timers.js:198:5)

This PR also ensures smoother upgrades: when upgrading, if admins do not set webirc to null or something, it will just refuse to run.