This is an unusual question but just in case you will need an answer. After
researching and you find out that everything is good, you can basically say
that you couldn't find anything wrong. Try to mix in humor if you can since
this is an odd question. Also, take a moment to pretend to be thinking about
the question before answering immediately.
The last sentence in the answer is a question you should say with a slight
smile. That should lighten the tension a little and show that you are fully
confident.