Default to admin translation domain (#4234)

This was the previous behavior, this fix restores the behavior while still not
using the deprecated AbstractAdmin::trans() method
Fixes #4230

Resources/views/CRUD/base_show.html.twig

@@ -83,7 +83,7 @@ file that was distributed with this source code.
                         <div class="box-header">
                             <h4 class="box-title">
                                 {% block show_title %}
-                                    {{ show_group.name|trans({}, show_group.translation_domain) }}
+                                    {{ show_group.name|trans({}, show_group.translation_domain|default(admin.translationDomain)) }}
                                 {% endblock %}
                             </h4>
                         </div>