MDL-76362 workshop: check for empty (null included) feedbackreviewer

As far as feedbackreviewer can be null, we cannot, since PHP 8.1
apply any string operation (trim, strlen...) on it, hence, checking
before applying.

mod/workshop/renderer.php

@@ -1841,7 +1841,7 @@ class mod_workshop_renderer extends plugin_renderer_base {
                         $output .= $this->render($workshop->prepare_submission_summary($submission, $shownames));
                         $output .= $this->box_end();
 
-                        if (strlen(trim($assessment->feedbackreviewer)) > 0) {
+                        if (!empty($assessment->feedbackreviewer) && strlen(trim($assessment->feedbackreviewer)) > 0) {
                             $output .= $this->render(new workshop_feedback_reviewer($assessment));
                         }
                     }