Based on Roddy's question about nobody volunteering for the exit row, I can imagine a related but more extreme scenario: what if there are no passengers on the plane who are eligible for an exit row seat? (Say if you had a charter flight full of seniors.) Do the exit rows simply remain empty in that case? How would the operator ensure that the overwing exits are usable in case they are needed?
In such an extreme case, the row would be unoccupied.
Nothing says that the row has to have people in it; I've had flights with under a dozen passengers, and if everybody had seats up front, the row would be empty, and that's okay.
You can't have unqualified people in the Emergency Exit row -- that's the limitation. Nothing requires the FA's the move qualified people into the row. That typically does happen (and as has been pointed out elsewhere, that row is often desirable due to legroom), but it's okay if the row remains empty.