Bug fix - call config value for `email_log.mailgun.filter_unknown_emails` properly

Bugfix.

IF check checking should we log events without parent email (by message ID) is now calling config value correctly.

You should not change this config value email_log.mailgun.filter_unknown_emails from default true to false at this point as there is a foreign key constraint set up on email_log_events table making it impossible to actually save the events which don't belong to any row in email_log.