I think "empty chairing" is the perfect opportunity for questions to be posed that will make the audience think very seriously about why the absentee interviewee wanted to make himself scarce. Of course, the strategy - the equivalent of the invariable "no comment" option of the career criminal - has been dreamed up by the Conservative campaign team but I'm hoping it will, at least to some extent, backfire.
Personally, I'm fed up with hearing Boris Johnson talking over interviewers and avoiding difficult questions by continually spouting "oven ready" "get Brexit done" "unleash Britain's potential", etc, etc, even when Brexit has no bearing on the question being asked.