MDL-47434 format_topics: section 0 navigation tweak

Section 0 is now only shown in the navigation block if it contains
activities.
If there are no activities then the section is removed.
This mimicks the display on the course page.

course/format/topics/lib.php

@@ -143,6 +143,19 @@ public function extend_course_navigation($navigation, navigation_node $node) {
 
         // check if there are callbacks to extend course navigation
         parent::extend_course_navigation($navigation, $node);
+
+        // We want to remove the general section if it is empty.
+        $modinfo = get_fast_modinfo($this->get_course());
+        $sections = $modinfo->get_sections();
+        if (!isset($sections[0])) {
+            // The general section is empty to find the navigation node for it we need to get its ID.
+            $section = $modinfo->get_section_info(0);
+            $generalsection = $node->get($section->id, navigation_node::TYPE_SECTION);
+            if ($generalsection) {
+                // We found the node - now remove it.
+                $generalsection->remove();
+            }
+        }
     }
 
     /**