MDL-76362 enrol_lti: Prevent calling to DataConnector with null keys

While it could have been fixed in DataConnector (3rd part lib), better
prevent in our code to call to it with null keys.

Covered by unit tests.

enrol/lti/classes/data_connector.php

@@ -88,16 +88,18 @@ public function loadToolConsumer($consumer) {
         global $DB;
 
         $id = $consumer->getRecordId();
+        $key = $consumer->getKey();
+        $result = false;
 
         if (!empty($id)) {
             $result = $DB->get_record($this->consumertable, ['id' => $id]);
-        } else {
-            $key256 = DataConnector::getConsumerKey($consumer->getKey());
+        } else if (!empty($key)) {
+            $key256 = DataConnector::getConsumerKey($key);
             $result = $DB->get_record($this->consumertable, ['consumerkey256' => $key256]);
         }
 
         if ($result) {
-            if (empty($key256) || empty($result->consumerkey) || ($consumer->getKey() === $result->consumerkey)) {
+            if (empty($key256) || empty($result->consumerkey) || ($key === $result->consumerkey)) {
                 $this->build_tool_consumer_object($result, $consumer);
                 return true;
             }