MDL-83956 course: update spacer icon for category management tree.

Prior to fd676913 the manage course categories tree element used
the 'i/empty' icon as a fixed width spacer (fa-fw). However this
icon is now shown as a square, which looks out of place here.

Change to a different icon that maps to the same fixed width spacer
in order to restore the previous behaviour.

course/classes/management_renderer.php

@@ -231,7 +231,7 @@ class core_course_management_renderer extends plugin_renderer_base {
             );
         } else {
             $icon = $this->output->pix_icon(
-                    'i/empty',
+                    'i/navigationitem',
                     '',
                     'moodle',
                     array('class' => 'tree-icon'));