SDC prop validation error on alert

Problem/Motivation

templates/misc/status-messages.html.twig:

{{ include('bootstrap_barrio:alert',
  { message_list: message_list,
    status_headings: status_headings,
    attributes: attributes
  })
}}

is triggering a fatal SDC validation error because message_list variable in the template can be NULL while in the component it can't.

Proposed resolution

Add default.