Lack of libidn silently breaks `jidprep()` and MUC occupant JIDs
The dependency on libidn is a soft one, however, the jidprep() function won't work if libidn isn't there (it will return "" instead of the prepped string).
This function is currently only used in XmppComponent::send_user_join() where it will make the jid attribute of MUC occupant elememnts vanish.
This is a ticking time bomb, waiting to create impossible-to-debug corner cases in your codebase.
Please do one of these:
- get rid of jidprep() altogether
- make jidprep() return the original string, which is slightly less bad than "".
- make libidn a hard requirement