Stop log spam because subtitleText was called without a valid event

It's up to the call site to check if the event is valid before calling
this function, and it prevents tons of log spam because we didn't check
yet.

src/models/roomlistmodel.cpp

@@ -349,7 +349,7 @@ QVariant RoomListModel::data(const QModelIndex &index, int role) const
         return m_categoryVisibility.value(data(index, CategoryRole).toInt(), true);
     }
     if (role == SubtitleTextRole) {
-        if (room->lastEventIsSpoiler()) {
+        if (room->lastEvent() == nullptr || room->lastEventIsSpoiler()) {
             return QString();
         }
         EventHandler eventHandler;