prevent php8-warnings, closes #3155

Closes #3155

Merge request studip/studip!2131

app/controllers/oer/endpoints.php

@@ -132,6 +132,7 @@ class Oer_EndpointsController extends StudipController
         }
 
         $output = ['results' => []];
+        if (isset($this->materialien)) {
             foreach ($this->materialien as $material) {
                 $data = [];
                 $data['host'] = [
@@ -166,6 +167,7 @@ class Oer_EndpointsController extends StudipController
                 }
                 $output['results'][] = $data;
             }
+        }
         $this->render_json($output);
     }