participant leaving a room is not noticed by participantStatusListeners

When a participant leaves a room, this is not noticed by the participantStatusListener.

I could fix this by this change

@@ -197,7 +197,7 @@ public class MultiUserChat {

                 case unavailable:

                     occupantsMap.remove(from);

                     MUCUser mucUser = MUCUser.from(packet);

-                    if (mucUser != null && mucUser.getStatus() != null) {

+                    if (mucUser != null && !mucUser.getStatus().isEmpty()) {

                         // Fire events according to the received presence code

                         checkPresenceCode(

                             mucUser.getStatus(),