None of 25 students at question session before exam has a question. What should lecturer do?
Some specific suggestions:


You likely know what material the students have been having issues with, based on questions from lecture, office hours, previous homework sets, or whatever. Mention those topics and review them.
Instead of asking for broad-based questions, go over the syllabus lecture by lecture, calling out specific topics by name and asking whether there was any issues with each of those. It'll help prime their memory.
Ask the TAs to attend. If that's not possible, ask them to send you common questions ahead of time. They also know what the students are finding difficult.