Questionnaire: Search indexing crashes out in error case

In questionnaire, classes/search/activity.php, it calls the base class get_document function. This function sometimes returns false if there is an error (such as invalid/partial data remaining in the database for a questionnaire), but the search class doesn't take account of this and can cause a PHP fatal error as a result, bringing search indexing (and the rest of that cron run) to a halt.

See attached patch.