fix: clear Mailwizz checkboxlist tag when deleting subscriber

Encode empty checkboxlist arrays as an empty scalar so multipart requests
include the field. On delete, PUT only coupon and tag fields to Mailwizz
after merging the tag CSV from getSubscriber.

Made-with: Cursor

app/Services/MailwizzService.php

@@ -202,7 +202,8 @@ final class MailwizzService
         $out = [];
         foreach ($data as $key => $value) {
             if (is_array($value)) {
-                $out[$key] = $value;
+                // Empty arrays are omitted by Laravel's multipart encoder (no KEY[] parts), so Mailwizz never clears checkboxlist fields.
+                $out[$key] = $value === [] ? '' : $value;
 
                 continue;
             }