It's a moot point in these days of high loads on every flight.

But no, it is absolutely not illegal to have an empty exit row. Take a hypothetical flight with only 1 pax onboard, who is 14 and doesn't speak English. Would the airline have to find someone just to fill that exit row because said pax isn't eligible to sit in it?

The FAs can handle opening the emergency exits just fine absent pax to help (or hinder, as the case may be).