Can a block ever be empty? The standard response is "no, the reward is at least one transaction." What about after there is no more reward?
That is far in the future and can still be changed, but at the current time a coinbase transaction is required and will always be present. It is very likely that this rule will be upheld in the future as well. It is also possible that the transaction accuracy could be increased enough for there to always be a block reward, however minuscule.