Fixes a null pointer call

It seems like the case is possible as we already are treating the case
in isUserBanned. Doesn't seem ideal as it shows "" where the username
should be but it's better than a crash.

src/libneochat/neochatroom.cpp

@@ -1660,7 +1660,11 @@ void NeoChatRoom::cleanupExtraEvent(const QString &eventId)
 }
 QString NeoChatRoom::invitingUserId() const
 {
-    return currentState().get<RoomMemberEvent>(connection()->userId())->senderId();
+    auto event = currentState().get<RoomMemberEvent>(connection()->userId());
+    if (!event) {
+        return {};
+    }
+    return event->senderId();
 }
 
 void NeoChatRoom::setRoomState(const QString &type, const QString &stateKey, const QByteArray &content)