Forgetting to pass --user_domain leads to a NPE

[linkmauve]

Here is the stack trace:

Exception in thread "main" java.lang.NullPointerException
        at java.util.Hashtable.put(Hashtable.java:459)
        at java.util.Properties.setProperty(Properties.java:166)
        at java.lang.System.setProperty(System.java:796)
        at org.jitsi.jicofo.Main.main(Main.java:166)