PHP 8: Warnungen in JSON-API Route der Courseware

Closes #4268

Merge request studip/studip!3309

lib/classes/JsonApi/Routes/Courseware/UserProgressesOfUnitsShow.php

@@ -115,10 +115,12 @@ class UserProgressesOfUnitsShow extends NonJsonApiController
 
         $usersCounter = count($courseMemberIds);
         foreach ($blks as $blk) {
+            if (isset($userProgresses[$blk])) {
                 $progresses = $userProgresses[$blk];
                 $usersProgress = $progresses['count'] ? (float) $progresses['grade'] : 0;
                 $data['progress'] += $usersCounter > 0 ? $usersProgress / $usersCounter : 0;
             }
+        }
 
         return $data;
     }