Fix no recipients formatter

Author: Ilaria Orlando

src/Backend/Modules/FormBuilder/Engine/Model.php

@@ -289,11 +289,17 @@ public static function existsIdentifier(string $identifier, int $ignoreId = null
      */
     public static function formatRecipients(string $string): string
     {
+        if ($string === '') {
+            return '';
+        }
+
+        $unserialized = @unserialize($string, ['allowed_classes' => false]);
+
         return implode(
             ', ',
             (array) array_map(
                 'htmlspecialchars',
-                @unserialize($string, ['allowed_classes' => false])
+                $unserialized
             )
         );
     }