MDL-48210 updates checker: Improve cron notifications processing

These are cosmetic improvements spotted while working on the issue.

1. Do not waste time if there are no changes to notify about.
2. Fix the legacy plugin manager's get_plugins() call.

lib/classes/update/checker.php

@@ -634,9 +634,13 @@ class checker {
     protected function cron_notifications(array $changes) {
         global $CFG;
 
+        if (empty($changes)) {
+            return;
+        }
+
         $notifications = array();
         $pluginman = \core_plugin_manager::instance();
-        $plugins = $pluginman->get_plugins(true);
+        $plugins = $pluginman->get_plugins();
 
         foreach ($changes as $component => $componentchanges) {
             if (empty($componentchanges)) {
@@ -696,6 +700,7 @@ class checker {
         global $CFG;
 
         if (empty($notifications)) {
+            $this->cron_mtrace('nothing to notify about. ', '');
             return;
         }