MDL-82359 Navigation: ignore navadduserpostslinks if no forum

The admin setting navadduserpostslinks (default on) causes debug
warnings due to missing language strings on every page while logged in
if forum is uninstalled.

This is not the worst problem if running without forum installed,
because you can just turn off the admin setting, but it makes it
impossible to run Behat tests.

This change means that the option does nothing if forum is uninstalled.
This is a small change just to make it work without significantly
altering anything.

lib/navigationlib.php

@@ -2589,7 +2589,8 @@ protected function load_for_user($user=null, $forceforcontext=false) {
                 }
             }
 
-            if (!empty($CFG->navadduserpostslinks)) {
+            // Only if the forum links are enabled and forum is installed...
+            if (!empty($CFG->navadduserpostslinks) && \core_component::get_component_directory('mod_forum') !== null) {
                 // Add nodes for forum posts and discussions if the user can view either or both
                 // There are no capability checks here as the content of the page is based
                 // purely on the forums the current user has access too.
@@ -5201,7 +5202,8 @@ protected function generate_user_settings($courseid, $userid, $gstitle='usercurr
             $profilenode = $mainpage->add(get_string('profile'), new moodle_url('/user/profile.php',
                     array('id' => $user->id)), self::TYPE_SETTING, null, 'myprofile');
 
-            if (!empty($CFG->navadduserpostslinks)) {
+            // Only if the forum links are enabled and forum is installed...
+            if (!empty($CFG->navadduserpostslinks) && \core_component::get_component_directory('mod_forum') !== null) {
                 // Add nodes for forum posts and discussions if the user can view either or both
                 // There are no capability checks here as the content of the page is based
                 // purely on the forums the current user has access too.