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/view.php

@@ -695,7 +695,7 @@ switch ($workshop->phase) {
                 echo $output->render($workshop->prepare_submission_summary($submission, $shownames));
                 echo $output->box_end();
 
-                if (strlen(trim($assessment->feedbackreviewer)) > 0) {
+                if (!empty($assessment->feedbackreviewer) && strlen(trim($assessment->feedbackreviewer)) > 0) {
                     echo $output->render(new workshop_feedback_reviewer($assessment));
                 }
             }