add fallback if `oauth_signature_method` is not set, fixes #1042

Closes #1042

Merge request studip/studip!612

lib/models/LtiData.php

@@ -111,7 +111,7 @@ class LtiData extends SimpleORMap
             return $this->tool->oauth_signature_method;
         }
 
-        return $this->options['oauth_signature_method'];
+        return $this->options['oauth_signature_method'] ?? 'sha1';
     }
 
     /**