Issue #3241275 by alexpott:...

Issue #3241275 by alexpott: \Drupal\user\Plugin\migrate\source\d6\User::prepareRow() causes deprecations on PHP 8.1

core/modules/user/src/Plugin/migrate/source/d6/User.php

@@ -68,7 +68,10 @@ public function prepareRow(Row $row) {
     }
 
     // Unserialize Data.
-    $row->setSourceProperty('data', unserialize($row->getSourceProperty('data')));
+    $data = $row->getSourceProperty('data');
+    if ($data !== NULL) {
+      $row->setSourceProperty('data', unserialize($row->getSourceProperty('data')));
+    }
 
     return parent::prepareRow($row);
   }