The failsafe I guess you could say is the fact that miners benefit from including transactions in a block as that means they get paid the fee (Assuming there is one). So fortunately there shouldn't be a situation where miners are deliberately mining transactionless blocks. Occasionally it happens by pure chance but that's just the random nature of block generation.

Of course I suppose you could argue that a miner might mine transactionless blocks in order to cut down on the block size for faster propagation, but that'd seem pretty counter productive.