Use \DateTime again in Import\Category

The refactor changed this code to use  Magento\Framework\Stdlib\DateTime,
but it's constructor isn't the same as \DateTime, so I reverted it.

\DateTime->new accepting null is deprecated, so I also fixed that warning

Author: rbrown

Model/Import/Category.php

@@ -954,13 +954,13 @@ private function saveCategories(): self
 
                     $time = !empty($rowData[CategoryModel::KEY_CREATED_AT])
                         ? strtotime($rowData[CategoryModel::KEY_CREATED_AT])
-                        : null;
+                        : 'now';
 
                     // entity table data
                     $entityRow = [
                         CategoryInterface::KEY_PARENT_ID => $parentCategory['entity_id'],
                         CategoryInterface::KEY_LEVEL => $parentCategory[CategoryInterface::KEY_LEVEL] + 1,
-                        CategoryInterface::KEY_CREATED_AT => (new DateTime($time))
+                        CategoryInterface::KEY_CREATED_AT => (new \DateTime($time))
                             ->format(DateTime::DATETIME_PHP_FORMAT),
                         CategoryInterface::KEY_UPDATED_AT => "now()",
                         CategoryInterface::KEY_POSITION => $rowData[CategoryInterface::KEY_POSITION]