When user delete liveblog post it doesn't trigger delete event for it

Fixed error on libeblog post deletion:

Error: Call to a member function id() on null in Drupal\liveblog_pusher\Plugin\LiveblogNotificationChannel\PusherNotificationChannel->triggerLiveblogPostEvent() (line 172 of modules/contrib/liveblog/modules/liveblog_pusher/src/Plugin/LiveblogNotificationChannel/PusherNotificationChannel.php).

Fixed more places.