Blocks aren’t required to contain transactions; the transactions included in a block are completely up to the miner, and in most cases any valid block will be quickly propagated by the miner to gain the coinbase reward since that far outweighs any reward from transactions at this point in time.
Mining consists of varying the contents of a block for every hash attempt; including hashing an empty block. If an otherwise valid empty block happens to hash to a value that fits the difficulty requirements, then it is a valid block.
