For info, I replaced
'ticket.message' => count($ticket->getThreads()) > 0 ? preg_replace("/<img[^>]+\>/i", "", $ticket->getThreads()->get(0)->getMessage()) : '',
by
'ticket.message' => (isset($ticket->createdThread)) ? $ticket->createdThread->getMessage() : ''
and now, the message is present in the mail when creating a ticket.
(in ./vendor/uvdesk/core-framework/Services/EmailService.php)