Fix PHP8 notice when adding categories

Author: svivian

qa-include/pages/admin/admin-categories.php

@@ -147,7 +147,7 @@
 			$errors['name'] = qa_lang_sub('main/max_length_x', QA_DB_MAX_CAT_PAGE_TITLE_LENGTH);
 		else {
 			foreach ($incategories as $category) {
-				if (!strcmp($category['parentid'] ?? '', $inparentid) &&
+				if (!strcmp($category['parentid'] ?? '', $inparentid ?? '') &&
 					strcmp($category['categoryid'], $editcategory['categoryid'] ?? '') &&
 					qa_strtolower($category['title']) == qa_strtolower($inname)
 				) {