Fix error when no recipients are given

When having chosen the database method no recipients are given resulting in an error from the formatRecipients function
Changed the function to only run when recipients are provided

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

@@ -285,17 +285,21 @@ public static function existsIdentifier(string $identifier, int $ignoreId = null
      *
      * @param string $string The serialized string that should be formatted
      *
-     * @return string
+     * @return string|null
      */
-    public static function formatRecipients(string $string): string
+    public static function formatRecipients(string $string): ?string
     {
-        return implode(
-            ', ',
-            (array) array_map(
-                'htmlspecialchars',
-                @unserialize($string, ['allowed_classes' => false])
-            )
-        );
+        if ($string) {
+            return implode(
+                ', ',
+                (array)array_map(
+                    'htmlspecialchars',
+                    @unserialize($string, ['allowed_classes' => false])
+                )
+            );
+        }
+
+        return null;
     }
 
     /**