Some clients (Gajim) allows to join a chat room with an empty nickname. They shouldn’t but anyway this happens now.

One of the issues I described here

But there are issues in the Spark itself that probably can be more or less easily fixed or cause less harm.

In the Spark I got and exception when leaving a chat in which I'm an admin with an empty nickname:

So the chat.getNickname() returns null and it's later is parsed with an error.
As far I understood the nickname is really needed for a chat.