Why is this block empty? The block #492972 mined today, seems to be totally empty (except the 12.5 BTC reward, i.e. 90'000 USD as of today). 

How can this happen?
Miners are free to mine empty blocks if they want. They do not HAVE to include transactions in them. Any block that is well-formatted and extends the blockchain is valid. Miners who do that (mining empty blocks) get the coinbase transaction (12.5BTC) and don't have to deal with the "hassle" of checking pending transactions. They lose the potential transaction fees though (much less than the reward).

Sure, if every miner did that, the bitcoin system would be over.