Accessibility: fix role for nested options (#5916)

src/js/select2/results.js

@@ -234,7 +234,8 @@ define([
       }
 
       var $childrenContainer = $('<ul></ul>', {
-        'class': 'select2-results__options select2-results__options--nested'
+        'class': 'select2-results__options select2-results__options--nested',
+        'role': 'none'
       });
 
       $childrenContainer.append($children);