Average viewership across 366 games in 2018 was about 1.8 million:
https://footballfoundation.org/news/2019/6/11/2018-19_Attendance_Release.aspx
The largest stadiums in the college football hold just a hair over 100,000 people, and I would guess those schools draw far more than the average on TV. So you're looking at like a 3-5% increase in TV viewers, max, based on zero attendance.
That's not to say advertising rates wouldn't change. So far, advertising spending has actually gone down. When people are unemployed (or just dealing with anxiety about how the world is changing), they're not going to be spending as much money. When consumers aren't spending money, companies aren't going to spend as much on advertising.
That could certainly change in the months ahead, but consumer behavior generally will have a far greater impact on advertising rates than a tiny change in viewership resulting from no fans in attendance.