Footer: Resolve PHP 8.1 warning.

resources/footer.php

@@ -163,7 +163,7 @@
 		else if (isset($_SESSION['software_name'])) {
 			$document_title = $_SESSION['software_name'];
 		}
-		$document_title = (!empty($document['title']) ? $document['title'].' - ' : null).$document_title;
+		$document_title = (!empty($document['title']) ? $document['title'].' - ' : null).($document_title ?? '');
 		$view->assign('document_title', $document_title);
 	//domain selector control
 		$domain_selector_enabled = permission_exists('domain_select') && count($_SESSION['domains']) > 1 ? true : false;