ensure data is present before using it in Shib auth plugin, fixes #4616

Closes #4616

Merge request studip/studip!3429

lib/classes/auth_plugins/StudipAuthShib.php

@@ -42,6 +42,7 @@ class StudipAuthShib extends StudipAuthSSO
 
             $this->userdata = json_decode($auth, true);
 
+            if ($this->userdata) {
                 if ($this->username_attribute !== 'username') {
                     $this->userdata['username'] = $this->userdata[$this->username_attribute];
                 }
@@ -51,6 +52,7 @@ class StudipAuthShib extends StudipAuthSSO
                 }
             }
         }
+    }
 
     /**
      * Return the current username.