Fix open_basedir warning when creating a menu item (joomla#36789)

administrator/components/com_menus/models/item.php

@@ -1145,7 +1145,7 @@ protected function preprocessForm(JForm $form, $data, $group = 'content')
 						$formFile = $path;
 					}
 				}
-				else
+				elseif ($base)
 				{
 					// Now check for a component manifest file
 					$path = JPath::clean($base . '/metadata.xml');