Update AuditTrailRepository to improve user handling and fix nullable user type in AuditTrail

- Changed `getUser` method in `AuditTrail` to allow null values by updating the return type to `?User`.
- Updated `insert` method in `AuditTrailRepository` to handle nullable users, ensuring the `user_id` parameter is set only when applicable.
- Improved code robustness by adding null-check logic for user assignment in the SQL query.

src/Bundle/ChillMainBundle/Entity/AuditTrail.php

@@ -54,7 +54,7 @@ class AuditTrail
         return $this->id;
     }
 
-    public function getUser(): User
+    public function getUser(): ?User
     {
         return $this->user;
     }