[DX] Backward compatibility in theme_item_list()

Description of the need

Since 1.27.0 added the 'wrapper_attributes' parameter in the $variable array in core/includes/theme.inc > function theme_item_list($variables). We should check the existence of this parameter if we want backward compatibility for the contrib modules.

Proposed solution

Now:

function theme_item_list($variables) {
  ...
  $wrapper_attributes = $variables['wrapper_attributes'];

Fixed:

$wrapper_attributes = isset($variables['wrapper_attributes']) ? $variables['wrapper_attributes'] : array();

Additional information

Similar situation with the 'empty' parameter since 1.21.0.

$empty= isset($variables['empty']) ? $variables['empty'] : '';