It has nothing to do with the miner that never collected the reward, it's just a miner that has mined an empty block. Even if the mempool is full a block may contain no transactions and still be valid. It happens from time to time, unfortunately. The single transaction within the block is simply the block reward as you very well observed.
To be more specific, empty blocks occur shortly after a block was found, because miners haven't assembled a new block of transactions yet, but it would be a waste of time to just sit and wait for it to happen, so they start with mining empty blocks after they discover new blocks on the network. The probability of it happening is quite low, but it still happens from time to time.
Also, slush pool has made an official statement that they will never mine an empty block if there are transactions in mempool.
