MDL-69751 course: Handle missing guest user context

Makes sure the guest user context exists when fetching
the recommended modules. If the context does not exist
an exception is being thrown describing the problem.

course/classes/local/service/content_item_service.php

@@ -110,7 +110,12 @@ private function get_recommendations(): array {
             throw new \coding_exception('The guest user does not exist in the database.');
         }
 
-        $favourites = $this->get_content_favourites(self::RECOMMENDATION_PREFIX, \context_user::instance($CFG->siteguest));
+        // Make sure the guest user context exists.
+        if (!$guestusercontext = \context_user::instance($CFG->siteguest, false)) {
+            throw new \coding_exception('The guest user context does not exist.');
+        }
+
+        $favourites = $this->get_content_favourites(self::RECOMMENDATION_PREFIX, $guestusercontext);
 
         $recommendationcache->set($CFG->siteguest, $favourites);
         return $favourites;