MDL-71909 assignfeedback_editpdf: Do not queue conversions when not on

Before this change conversion tasks would be queued for assignments that
did not have the pdf annotation method enabled.

The change will also prevent queing when the plugin is disabled.

mod/assign/feedback/editpdf/classes/event/observer.php

@@ -51,6 +51,14 @@ class observer {
      * @param \mod_assign\event\base $event The submission created/updated event.
      */
     protected static function queue_conversion($event) {
+        $assign = $event->get_assign();
+        $plugin = $assign->get_feedback_plugin_by_type('editpdf');
+
+        if (!$plugin->is_visible() || !$plugin->is_enabled()) {
+            // The plugin is not enabled on this assignment instance, so nothing should be queued.
+            return;
+        }
+
         $data = [
             'submissionid' => $event->other['submissionid'],
             'submissionattempt' => $event->other['submissionattempt'],