Prevent division by zero warning, fixes #2437

Closes #2437

Merge request studip/studip!1630

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

@@ -117,7 +117,7 @@ class UserProgressesOfUnitsShow extends NonJsonApiController
         foreach ($blks as $blk) {
             $progresses = $userProgresses[$blk];
             $usersProgress = $progresses['count'] ? (float) $progresses['grade'] : 0;
-            $data['progress'] += $usersProgress / $usersCounter;
+            $data['progress'] += $usersCounter > 0 ? $usersProgress / $usersCounter : 0;
         }
 
         return $data;