It's an interesting hypothetical. Here's how I run the course. I have lectures for each topic (the lectures I've created). Accompanying the lectures are problem sets along with solutions. Students are expected to watch the lecture and attempted some of the problems before coming to class. In class I pass out a problem set. Usually 30 to 40 problems.

At the beginning of the semester I have around 35 students in the class. When I pass out the problem set I go around helping students with the problems. They typically get into groups of 3 or 4. Since I have a lot of students I can't waste my time on someone who didn't bother to even watch the lecture. It's not fair to those who did watch the lecture and need help.

I've never encountered a situation in which no one watched the lecture. I don't know what my response would be if that happened.