Drag drop questions don't validate that drop zones have been defined (causing division by zero errors in the statistics)

I worked out the real problem here. To reproduce.

1. Go to the question bank.
2. Choose 'Create question' and select Drag-drop into image.
3. Set the Question name, Question text and upload a background image.
4. Click Save changes.

Expected result: You get validation errors telling you that you must define at least on drag item.

Actual result: The question is saved, even though it cannot possibly work.

The errors when computing the statistics are just a side-effect of this, but since people have bad quesitons in their databases, we need to make that calculation robust too.

Original bug report follows:

This task fails to run in our Moodle 4.1dev+ environment (PHP 8.0, Redis 6, Apache 2.4, MariaDB 10.5):

Execute scheduled task: Vraagstatistieken opnieuw berekenen (quiz_statistics\task\recalculate)
... started 13:30:02. Current memory use 5.4 MB.
Debugging increased temporarily due to faildelay of 1920
... used 15169 dbqueries
... used 9.6491088867188 seconds
Scheduled task failed: Vraagstatistieken opnieuw berekenen (quiz_statistics\task\recalculate),Division by zero
Backtrace:

• line 151 of /question/type/ddimageortext/questiontypebase.php: call to qtype_ddtoimage_question_base->get_random_guess_score()
• line 471 of /question/classes/statistics/questions/calculator.php: call to qtype_ddtoimage_base->get_random_guess_score()
• line 82 of /question/classes/statistics/questions/calculator.php: call to core_question\statistics\questions\calculator->get_random_guess_score()
• line 642 of /mod/quiz/report/statistics/report.php: call to core_question\statistics\questions\calculator->__construct()
• line 941 of /mod/quiz/report/statistics/report.php: call to quiz_statistics_report->get_all_stats_and_analysis()
• line 83 of /mod/quiz/report/statistics/classes/task/recalculate.php: call to quiz_statistics_report->calculate_questions_stats_for_question_bank()
• line 259 of /lib/cronlib.php: call to quiz_statistics\task\recalculate->execute()
• line 120 of /lib/cronlib.php: call to cron_run_inner_scheduled_task()
• line 73 of /lib/cronlib.php: call to cron_run_scheduled_tasks()
• line 178 of /admin/cli/cron.php: call to cron_run()