So, if the pool has no buffer funds
Regardless of your already answered question, the FAQ also states this:
What happens with the surplus BTC generated and the queue is empty?
Surplus BTC are placed in the pool's reserve balance (aka buffer or "rainy day fund"). On unlucky runs, the pool may use these funds to manually payout older balances. This may be done solely at the pool's discretion.
So methinks we don't really know if the pools buffer is used up or not.