fix: typage comment insert card_responses

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/modules/cards/cards.service.ts

@@ -208,7 +208,7 @@ export class CardsService {
       id_card: card.id,
       id_utilisateur: userId,
       action: dto.action,
-      comment: dto.comment?.trim() || null,
+      comment: dto.comment?.trim() || undefined,
     });
 
     if (card.id_absence) {