Nothing at stake has been handled in 3 different ways,
1. Have a Coin with a Billion or more and this is never a problem.
2. Have extremely low minimum age , before the coins can stake again between 2 and 24 hours .
3. Run a Hybrid PoS/PoW coin, where PoW make up for any lack of stakers or PoS makes up for any lack of PoW miners.
That is not the "Nothing at stake" problem. It's not about "nobody is staking". Nothing at stake is a much more complex challenge: There are situations where "naive" PoS algorithms allow the participants to "stake" in several chains at the same time with no cost for the "staker", what makes possible an attack.
Nevertheless, a N@S attack has never been successfully realized in the larger PoS currencies (NXT, Peercoin, BTS, Blackcoin ...). NXT has a "reorg-prohibition" that handles the problem successfully. Newer Peercoin versions and their clones have also mechanisms that disincentive staking on several chains.